Engine Oil Explained: Viscosity, Additives, Cleaning and Racing Oil vs Street Oil, how to choose the correct engine oil

Engine oil is one of the most important components inside an engine, yet it is also one of the most misunderstood.

how to choose the correct engine oil

Drivers regularly choose oil based on brand reputation, price, viscosity or the words “racing” and “high performance” printed on the container. Some believe that thicker oil automatically provides better protection. Others assume that the most expensive competition oil must be the best choice for every performance car.

Neither assumption is correct.

The correct engine oil must suit the engine’s internal clearances, oil-pump design, operating temperature, power output, emissions equipment, driving conditions and intended service interval.

An oil developed for a racing engine that is rebuilt regularly may be unsuitable for a road-going supercar expected to start from cold, operate in traffic, protect catalytic converters and remain in service for thousands of kilometres.

Likewise, an ultra-low-viscosity road oil selected primarily for fuel economy may not be appropriate for an older competition engine with large bearing clearances and sustained high oil temperatures.

The objective is not to find the “best oil” in general. It is to find the correct oil for the engine and how it is being used.

What Does Engine Oil Actually Do?

The most obvious function of engine oil is lubrication. It creates a protective film between moving components and helps prevent direct metal-to-metal contact.

However, modern engine oil must perform several jobs simultaneously.

It must:

• Lubricate bearings, camshafts, pistons and valvetrain components

• Carry heat away from heavily loaded parts

• Control friction

• Keep contaminants suspended

• Prevent sludge and deposits

• Neutralise combustion acids

• Protect against corrosion

• Maintain hydraulic pressure

• Operate at cold-start temperatures

• Resist thinning at high temperature

• Resist oxidation

• Control foaming and aeration

• Protect turbochargers

• Remain compatible with seals

• Protect catalytic converters and particulate filters

  
  

This is why engine oil should be understood as an engineered component rather than a simple liquid lubricant.

Start With the Manufacturer’s Specification

The first step when choosing an engine oil is not selecting the brand or viscosity. It is identifying the exact specification required by the vehicle manufacturer.

Depending on the vehicle, the handbook may specify an approval such as:

• Porsche A40 or C40

• Mercedes-Benz MB 229.5, 229.51 or 229.52

• BMW Longlife specifications

• Volkswagen VW 504 00, 507 00 or another VW standard

• Renault RN specifications

• Ford WSS specifications

• General Motors dexos

• An ACEA category

• An API service category

  
  

The exact specification matters because two oils with the same viscosity can have completely different additive packages, high-temperature performance, emissions compatibility and engine-test results.

For example, two products may both be labelled 5W-30, but one may be designed for a modern diesel engine with a particulate filter while the other is intended for a petrol engine without the same aftertreatment requirements.

The viscosity alone does not confirm suitability.

API itself instructs owners to follow the vehicle manufacturer’s viscosity and performance recommendations. ACEA also warns that certain low-viscosity categories are unsuitable for engines not specifically designed for them.

What Does 0W-40 or 5W-30 Mean?

The numbers printed on the oil container describe the SAE viscosity grade.

In a multigrade oil such as 0W-40:

• 0W describes its low-temperature or winter performance.

• 40 describes its high-temperature viscosity classification.

The W means winter. It does not mean weight.

A 0W oil generally flows and pumps more easily at very low temperatures than a 5W or 10W product within the same type of application. This can help the oil reach bearings, camshafts and turbocharger components more quickly during a cold start.

The second number describes the viscosity range after the oil is hot. A 40-grade oil remains more viscous at operating temperature than a comparable 30-grade oil.

However, the numbers do not describe the quality of the oil.

SAE J300 defines engine-oil viscosity grades only in rheological terms. It does not evaluate deposit control, wear protection, oxidation resistance, catalyst compatibility or overall product quality.

A correctly approved 0W-40 may therefore provide substantially better protection than an unsuitable or low-quality 10W-60, despite the thicker product appearing more impressive on the label.

Thicker Oil Is Not Automatically Better

A thicker oil can create a stronger lubricating film under certain high-temperature and high-load conditions. It can also help maintain oil pressure in an engine with larger bearing clearances.

But increasing viscosity also has disadvantages.

Oil that is too thick may:

• Flow more slowly during cold starts

• Take longer to reach remote lubrication points

• Increase pumping losses

• Reduce fuel efficiency

• Affect variable valve timing

• Affect hydraulic tappets

• Raise oil pressure without improving actual oil flow

• Increase oil temperature in some conditions

• Reduce power through additional drag

  
  

Oil pressure and oil flow are related, but they are not the same thing.

High pressure caused by excessive viscosity does not necessarily mean that the bearings are receiving an ideal volume of oil. In some engines, the pressure-relief valve may open and divert flow while the dashboard still displays a reassuring pressure figure.

The correct viscosity should maintain sufficient pressure and film strength while still allowing adequate circulation and cooling.

What Is HTHS Viscosity?

For high-performance engines, the viscosity printed on the front of the bottle does not tell the complete story.

HTHS means high-temperature, high-shear viscosity. It measures how the oil behaves under conditions intended to represent heavily loaded areas such as bearings when the oil is hot and exposed to rapid shearing forces.

HTHS is especially relevant to:

• Main and connecting-rod bearings

• Camshaft contact areas

• Turbocharger bearings

• High-load track use

• Engines with high cylinder pressure

• Engines operating at sustained high oil temperatures

  
  

ACEA categories contain specific HTHS requirements. For example, ACEA C3 oils require a minimum HTHS viscosity of 3.5 mPa·s, while fuel-economy-focused C5 and C6 oils permit lower HTHS levels in engines specifically designed and approved for them. The newer C7 category permits an even lower minimum level and must only be used where the manufacturer has designed the engine for it.

This does not make a lower-HTHS oil poor quality. It means the oil and engine have been designed around a different compromise between protection, pumping losses and efficiency.

Using a high-HTHS oil in every engine is not automatically correct. Using a low-HTHS oil in an engine not designed for it can also be a serious mistake.

What Is Inside Engine Oil?

Most of the product is base oil. The remaining portion is an additive package carefully balanced to provide the required performance.

The additive package may contain:

• Detergents

• Dispersants

• Anti-wear agents

• Antioxidants

• Friction modifiers

• Viscosity-index improvers

• Corrosion inhibitors

• Pour-point depressants

• Anti-foam agents

• Seal-compatible components

• 
  

The precise formulation is normally proprietary. Knowing how much zinc or detergent an oil contains does not provide enough information to judge the entire product.

Engine oils are systems. The additives must work together without creating excessive deposits, foaming, catalyst contamination or chemical competition.

Detergents: More Than Simply Cleaning

Engine-oil detergents do not clean in the same way as household detergent.

They are generally used to control deposits and neutralise acidic by-products created during combustion and oil oxidation. Many detergent components are based on calcium or magnesium chemistry.

Detergents help prevent deposits from attaching themselves to hot surfaces such as pistons, ring grooves and valvetrain components.

They also contribute to the oil’s alkaline reserve, often represented by its total base number, or TBN. This reserve helps neutralise acids during the oil’s service life.

An oil intended for long road-service intervals may require substantial detergent capacity because it must remain effective through repeated cold starts, moisture contamination, combustion by-products and extended operation.

Dispersants: Keeping Contamination Suspended

Dispersants surround small contaminant particles and help prevent them from joining together to form sludge or deposits.

Instead of allowing soot, oxidation products and other debris to accumulate on engine components, dispersants help keep them suspended within the oil until they can be captured by the filter or removed during an oil change.

Lubrizol describes engine-oil dispersants as critical additives that help keep harmful debris suspended and prevent it from thickening the oil, causing wear or blocking the filter.

A dark oil is therefore not automatically a failed oil.

In some engines, darkening can indicate that the detergents and dispersants are carrying contamination rather than allowing it to remain attached to the inside of the engine.

Colour alone cannot establish the condition of an oil.

Anti-Wear Additives and ZDDP

One of the best-known anti-wear additives is ZDDP, or zinc dialkyldithiophosphate.

ZDDP contains zinc and phosphorus and forms a sacrificial protective layer on metal surfaces under conditions where the normal lubricating film becomes extremely thin.

It can be particularly important in engines with:

• Flat-tappet camshafts

• High valve-spring pressure

• Aggressive camshaft profiles

• High contact loads

• Older valvetrain designs

• Competition use

  
  

However, more zinc is not always better.

High phosphorus levels can affect the durability and efficiency of catalytic converters when oil is consumed and its additive components enter the exhaust system.

Modern road oils therefore balance anti-wear protection against emissions-system compatibility. Some competition-only oils use substantially higher zinc and phosphorus levels because protecting a heavily loaded racing valvetrain is prioritised over long-term catalytic-converter durability. Mobil, for example, states that its dedicated racing oils contain approximately twice the zinc level of its current low-viscosity API-approved passenger-car oil and are not recommended for normal street use.

This does not mean all road oils have insufficient anti-wear protection. Modern oils use complete additive systems and must pass defined wear tests rather than relying only on a high zinc figure.

Antioxidants

Heat and oxygen gradually degrade the oil.

Oxidation can increase viscosity and contribute to the formation of sludge, varnish and acidic compounds. Turbocharged engines place particular demands on oxidation resistance because the oil is exposed to high temperatures around the turbocharger bearings and oil galleries.

Antioxidants slow this degradation and help the oil retain its properties for longer.

Lubrizol notes that antioxidant additives increase oil life by reducing breakdown that can lead to sludge formation.

A racing engine may expose its oil to extreme temperatures over a short period. A road engine may expose the oil to less extreme temperatures but require it to resist oxidation for many months and thousands of kilometres.

These are different design requirements.

Viscosity-Index Improvers

Oil becomes thinner as it heats up.

Viscosity-index improvers are polymers that help a multigrade oil maintain the required viscosity across a wider temperature range.

They allow an oil to provide suitable cold-start performance while still retaining the necessary hot viscosity.

However, these polymers can be subjected to mechanical shearing inside the engine. Gear drives, oil pumps, bearings and valvetrain components can gradually break down some viscosity modifiers, causing the oil to lose viscosity during service.

A high-quality oil must therefore demonstrate appropriate shear stability and remain within its specified grade.

ACEA testing includes requirements for oils to remain “stay in grade” after defined shear testing.

Friction Modifiers

Friction modifiers reduce friction between certain moving surfaces and can improve fuel economy and efficiency.

They may also contribute to a small reduction in mechanical losses and therefore release a small amount of power.

However, the additive chemistry must suit the application.

A passenger-car oil containing friction modifiers should not automatically be used in a motorcycle where the engine, gearbox and wet clutch share the same oil. Certain friction modifiers can interfere with clutch operation and cause slipping.

Dedicated motorcycle oils are formulated around this shared-sump requirement. ExxonMobil, for example, states that its 4T motorcycle oil avoids the friction modifiers that could cause wet-clutch slippage and uses additive chemistry intended for the engine, gearbox and clutch.

Anti-Foam and Aeration Control

Oil containing air bubbles cannot support loads or operate hydraulic components as effectively as properly deaerated oil.

At high engine speeds, the crankshaft, oil pump and returning oil can introduce air into the lubricant. Poor sump control, overfilling or an unsuitable formulation can make the problem worse.

Anti-foam additives help control surface foam, while the complete formulation must also allow entrained air to separate from the oil.

This is particularly important in high-revving engines, dry-sump systems and vehicles exposed to sustained cornering loads.

The Compromise Between Lubricating and Cleaning

It is common to describe engine oil as making a compromise between lubrication and cleaning, but the relationship is more complicated than one working against the other.

Keeping the engine clean is part of protecting it from wear.

Sludge can restrict oil galleries and pickup screens. Piston deposits can affect ring movement. Contaminants can increase abrasive wear. Oxidised oil can lose its intended viscosity.

Detergents and dispersants therefore support lubrication by keeping critical surfaces and oil passages in better condition.

The real compromise is between the many requirements placed on the complete formulation:

• Wear protection

• Deposit control

• Acid neutralisation

• Friction reduction

• High-temperature film strength

• Fuel economy

• Cold-start flow

• Long service intervals

• Catalyst compatibility

• Particulate-filter compatibility

• Seal compatibility

• Low oil consumption

Increasing one additive does not automatically improve the complete oil. Additives can compete for metal surfaces, affect friction, contribute to ash or create other formulation challenges.

The best oil is not the one containing the highest concentration of every additive. It is the oil with the correct balance for the application.

What Does Low SAPS Mean?

SAPS stands for:

• Sulphated ash

• Phosphorus

• Sulphur

  
  

Modern petrol and diesel vehicles frequently use low- or mid-SAPS oils to protect exhaust aftertreatment systems such as:

• Three-way catalytic converters

• Diesel particulate filters

• Gasoline particulate filters

  
  

Some detergent and anti-wear additives produce ash when the oil is burned. This ash cannot be regenerated out of a particulate filter in the same way as ordinary soot and can gradually contribute to permanent filter loading.

Phosphorus can also affect catalytic-converter performance.

ACEA C-category oils are formulated for compatibility with catalysts, gasoline particulate filters and diesel particulate filters. ACEA specifically warns that the correct category must still be matched to the engine because the different C specifications are not universally interchangeable.

This is why replacing an approved low-SAPS road oil with a high-zinc competition oil may have consequences beyond the engine itself.

Modern Street Oils Have a Difficult Job

A high-quality street oil must operate across an extremely broad range of conditions.

A road car may experience:

• Repeated cold starts

• Short journeys

• Long periods of idling

• Stop-start operation

• Fuel dilution

• Moisture contamination

• Traffic in high ambient temperatures

• Occasional full-throttle use

• Extended motorway driving

• Long oil-change intervals

• Catalytic converters and particulate filters

  
  

Modern turbocharged direct-injection engines also create concerns such as low-speed pre-ignition, timing-chain wear and turbocharger deposits.

The current API SQ and ILSAC GF-7 standards, introduced in March 2025, include requirements addressing fresh- and aged-oil LSPI, timing-chain wear, piston and turbocharger deposits, sludge, varnish, low-temperature pumpability and emissions-system protection.

Street oil is therefore not a weaker form of racing oil.

It is designed to protect the engine and emissions equipment across a much wider operating window and usually for a much longer service period.

What Is Different About Racing Oil?

A genuine racing oil is formulated around competition conditions.

The engine may be:

• Fully warmed before high load

• Operated mainly at high RPM

• Exposed to sustained oil temperatures

• Using large bearing clearances

• Running high valve-spring pressure

• Producing very high cylinder pressure

• Rebuilt or inspected regularly

• Filled with fresh oil before each event

• Operating without road-legal emissions equipment

The oil may prioritise:

• High-temperature film strength

• Anti-wear performance

• Friction reduction

• Power output

• Aeration control

• Resistance to temporary fuel dilution

• Protection under extreme load

Some competition oils use elevated anti-wear chemistry and are not designed around long catalytic-converter life or extended road-drain intervals.

Mobil describes its dedicated Racing 0W-30 and 0W-50 products as track-only oils that are not recommended for street use, despite also claiming high-temperature protection, deposit control and engine cleanliness.

This illustrates an important point: a racing oil may still clean and protect extremely well, but its overall formulation may not be suitable for the requirements of a road vehicle.

Racing Oil Is Not Always Thicker

The term “racing oil” does not define a viscosity.

Competition oils are available in thin and thick grades. The correct viscosity depends on:

• Bearing clearances

• Oil temperature

• Oil-pump capacity

• Engine speed

• Power output

• Fuel type

• Intended race duration

• Engine-builder requirements

A purpose-built qualifying engine may use a relatively thin oil to reduce friction and increase power, accepting that the oil will be replaced immediately.

An endurance engine may use a different formulation to maintain viscosity and oxidation resistance over many hours.

An older race engine with larger bearing clearances may require a substantially thicker grade.

The word racing on the bottle is therefore not enough information to choose the oil.

Racing Oil and Track-Capable Street Oil Are Not the Same

Some oils are genuine competition-only products.

Others are road-approved performance oils that can also handle track use.

A street-and-track oil may offer:

• Correct manufacturer approvals

• Catalyst and GPF compatibility

• Strong oxidation resistance

• Active cleaning additives

• Cold-start protection

• Road-suitable drain intervals

• Sufficient high-temperature performance for occasional track use

For a road-registered Porsche, Ferrari, Lamborghini, McLaren, BMW M or Mercedes-AMG that attends occasional track days, an approved high-performance street oil is often a more sensible choice than a competition-only product.

The oil interval can then be reduced according to track use, oil temperature, fuel dilution and the vehicle manufacturer’s severe-service recommendations.

Using racing oil in a road car simply because it sounds superior is not professional oil selection.

When Does Competition Oil Make Sense?

A dedicated racing oil may be appropriate when:

• The engine is used exclusively for competition

• The catalytic converters or particulate filters are not part of the application

• The engine builder specifies the product

• Bearing clearances have been selected around its viscosity

• Oil temperature and pressure are monitored

• The oil is replaced frequently

• The engine is inspected regularly

• The required anti-wear package exceeds normal road-oil requirements

• The vehicle operates mainly at full load and high RPM

Even then, the oil should be selected through testing rather than reputation.

Oil pressure, temperature, consumption, filter inspection and used-oil analysis provide more useful information than brand loyalty or paddock rumours.

Choosing Oil for a Modified Engine

A modified engine may no longer operate under the conditions assumed by the manufacturer.

Increased boost pressure, higher torque, larger turbochargers, altered fuelling and sustained track use can increase:

• Bearing load

• Piston temperature

• Turbocharger temperature

• Oil temperature

• Fuel dilution

• Blow-by

• Oxidation

• Oil consumption

However, modification does not automatically mean that a very thick oil should be installed.

The correct decision should consider:

• Original and modified power output

• Bearing clearances

• Hot idle pressure

• Pressure at high RPM

• Maximum oil temperature

• Type of use

• Fuel type

• Oil-cooler capacity

• Turbocharger specification

• Engine-builder recommendation

• Oil-analysis results

Where the original manufacturer permits more than one viscosity, the more suitable approved grade can be selected for the climate and operating conditions.

Moving outside the approved range should be an informed technical decision, not a guess.

Oil Temperature Is Critical

A viscosity that performs correctly at 100°C may behave very differently when the oil reaches 125°C, 135°C or more during track use.

As temperature rises, viscosity falls.

This is why a car that shows acceptable oil pressure during road driving may display much lower pressure after several hard laps.

Before changing oil viscosity, the technician should establish:

• Actual oil temperature

• Pressure at idle

• Pressure at different engine speeds

• Whether the oil is aerating

• Whether the oil level is correct

• Whether the oil cooler is functioning

• Whether fuel dilution is present

• Whether bearing wear already exists

Changing to a thicker oil can sometimes be appropriate, but it should not be used to hide a mechanical problem or inadequate oil cooling.

Oil Change Intervals for Track Use

A manufacturer’s normal road-service interval is not automatically appropriate for repeated track driving.

Track use exposes the oil to:

• Higher average temperature

• Greater shear

• More blow-by

• More fuel contamination

• Increased oxidation

• Sustained turbocharger load

• Higher bearing loads

A road car used on circuit should normally have its oil level checked before and during the event, particularly if the engine is known to consume oil.

The replacement interval should be shortened according to the severity of use.

A single gentle track session is not equivalent to repeated high-temperature running on slick tyres. Service intervals should reflect the real conditions rather than simply counting kilometres.

Used-Oil Analysis

Used-oil analysis can provide valuable information when selecting oil for a high-value or heavily modified engine.

A laboratory can evaluate factors including:

• Viscosity change

• Fuel dilution

• Wear metals

• Coolant contamination

• Silicon contamination

• Oxidation

• Additive depletion

• Water content

• Total base number

One sample does not always provide a complete diagnosis. Trends across several oil changes are generally more useful.

Analysis can help determine whether the viscosity remains stable, whether the change interval is appropriate and whether abnormal wear or contamination is developing.

For an expensive competition engine, data is more reliable than assuming that the oil is suitable because it appears clean or maintains good pressure.

Should Aftermarket Oil Additives Be Used?

In most modern engines, additional aftermarket oil additives are unnecessary and can upset the balance of a properly formulated oil.

Adding extra zinc, friction modifier, viscosity improver or detergent changes the chemistry without testing the final mixture.

Possible consequences include:

• Additive competition

• Increased ash

• Catalyst contamination

• Foaming

• Changed friction characteristics

• Filter deposits

• Altered viscosity

• Reduced oil performance

A correctly selected oil already contains a balanced additive package.

An additional product should only be used where there is a specific, technically supported reason—not because more additive sounds safer.

Common Engine-Oil Mistakes

Choosing only by viscosity

The SAE grade describes viscosity, not the complete performance level or manufacturer approval.

Assuming thicker always protects better

Excessive viscosity can reduce cold flow, increase pumping losses and interfere with hydraulic systems.

Assuming racing oil is always superior

Competition oil may sacrifice emissions compatibility and extended service performance for a specialised operating environment.

Choosing oil by zinc level alone

Zinc is only one part of the anti-wear system and does not describe oxidation resistance, cleanliness, shear stability or catalyst compatibility.

Ignoring the particulate filter

A high-SAPS oil can contribute to permanent DPF or GPF ash loading if used in an incompatible engine.

Extending the interval because the oil is expensive

Premium oil still becomes contaminated and degraded. Price does not prevent fuel dilution, oxidation or additive depletion.

Using oil pressure as the only measurement

High pressure does not automatically confirm adequate flow, correct temperature or healthy bearings.

Mixing different additives and oil types

Most oils can tolerate limited mixing in an emergency, but deliberately creating an untested blend is not a professional calibration strategy.

How to Choose the Correct Engine Oil

A reliable selection process follows a clear order.

1. Identify the exact manufacturer requirement

Check the handbook, workshop information or official technical data for the correct specification and permitted viscosity grades.

2. Confirm the engine and emissions equipment

Establish whether the vehicle has a catalytic converter, DPF, GPF or other system requiring a low- or mid-SAPS oil.

3. Consider the type of use

Daily road use, short journeys, motorway driving, occasional track days and competition use impose different demands.

4. Consider the climate

Cold-start requirements and normal ambient temperature influence the appropriate winter viscosity.

5. Measure operating conditions

For modified or track-driven cars, record actual oil temperature and pressure rather than estimating them.

6. Check genuine specifications and approvals

Do not rely only on descriptions such as “recommended for,” “race technology” or “high performance.” Confirm that the product carries the exact required specification or approval.

7. Set the correct change interval

The interval should reflect mileage, time, fuel dilution, temperature, track use and the engine’s mechanical condition.

8. Inspect and analyse where necessary

Oil-filter inspection and used-oil analysis are valuable for expensive, modified or competition engines.

Our View at Torque Tuning

There is no universal engine oil that is correct for every high-performance car.

A road-going supercar requires an oil that protects the engine from cold start to full operating temperature, controls deposits, supports the turbochargers and remains compatible with its catalytic converters and particulate filters.

A competition engine may require a different balance, prioritising high-load film strength, anti-wear performance and reduced friction over long service intervals and road emissions compatibility.

The correct choice begins with the engine manufacturer’s specification. From there, the viscosity and product can be adapted to the climate, mechanical condition, modifications and real use of the vehicle.

At Torque Tuning, we do not select oil according to advertising or the thickest viscosity available.

We consider the complete application:

• Engine design

• Manufacturer approval

• Power level

• Bearing load

• Oil temperature

• Oil pressure

• Turbocharger requirements

• Exhaust aftertreatment

• Road or track use

• Intended change interval

Based in Marbella and serving owners across Puerto Banús, Sotogrande, Mijas and the wider Costa del Sol, Torque Tuning provides specialist servicing, diagnostics and performance solutions for premium and high-performance vehicles.

Our approach combines practical motorsport knowledge, detailed mechanical understanding and experience gained as a former McLaren test consultant.

The correct engine oil is not the product with the most impressive label. It is the oil that provides the right protection, cleanliness and stability for the exact engine and its operating conditions.