Engine & Gear Oil Explained: Their Purpose & Grading

Engine & Gear Oil: What Are They & What is The Grading?

A typical bottle of engine oil or gear oil can have a rather bewildering list of standards, grades and terms that can be confusing if you are unfamiliar with them. We have compiled this competition oils guide to cut through some of the jargon and help choose the most suitable lubricant for your motorsport car. 

Engine Oil Viscosity Grades Explained 

Engine Oils 

Viscosity is the most common parameter to choosing the correct oil. The viscosity of an oil is measured by its resistance to flow. This is determined by the volume of oil to flow over a set time (think of the difference in time filling a cup with water compared with custard) The Society of Automotive Engineers (SAE) has established a numerical code system for grading engine oils according to their viscosity characteristics known as SAE J300. There are two numbers that define viscosity meaning. The first number precedes the letter ‘W’ which stands for Winter. This measurement is related to how an oil flows when it is cold, such as at engine start-up. The second number is defined by how an oil flows at normal engine operating temperatures. 

The smaller the number, the better it will flow. A 5W-30 will flow easier than a 10W-30 at start-up temperatures and a 10W-30 will flow more easily than a 10W-40 at normal engine operating temperatures. Engine oil viscosity is important, oils naturally thicken as they cool and thin as they are heated. Thin, low viscosity oils give better protection to engine parts at cold temperature. Thick, high viscosity oils are typically better at maintaining film strength to protect engines at high temperatures. 

Some older / classic oils are referred to as a monograde and have a single viscosity rating such as 40 or 50. The motorsport monograde oils can be very effective if used in the correct operating temperature but tend to have very specific applications. 

For Motorsport Use

The ideal is to have easy and good flow characteristics to prevent damage on cold start up but very high film strength to maintain clearances at extremely high temperatures found in the competition. A low viscosity will also mean less power loss due to oil drag (sometimes a lower viscosity is used for qualifying before going back to the normal oil viscosity). It must be remembered the car manufacturers recommended viscosity is aimed at road use, and usually has a several viscosities for different climates / ambient temperatures.

However, for competition use the engine will run much hotter regardless of the ambient temperature so the higher viscosities listed are generally advised. However clever uses of additives in many oils mean a thinner oil can be often used with no loss in protection , or a standard weight oil can have much less friction than a basic budget oil of the same rating.

Care should be taken if changing the viscosity too far if the engine has variable valve timing, as the oil pressure can be too low to operate the timing system. These additives will be covered later in the blog. Classic competition cars can also take advantage of the massive advances in oil technology, engine building, and the improved quality of engine components. It is common for 1950’s or earlier engines to have a monograde oil specified in a single viscosity, one for winter and another for summer. Most of these engines will benefit with a swap from a monograde to a multigrade that will cover the range of both grades.

Gear Oil Grades and Numbers Explained

Gear Oils 

The SAE has a separate viscosity rating system for gear, axle, and manual transmission oils, SAE J306, which should not be confused with engine oil viscosity. The higher numbers of a gear oil (e.g., 75W-140) do not mean that it has higher viscosity than an engine oil. Interestingly a 90-grade gear oil viscosity falls between a 40 or 50 grade engine oil, and 80 grade gear oil is similar viscosity to a 20-grade engine oil. Multigrade SAE gearbox oil grades include two numbers separated by a ‘W’ (75W140, for instance), with the initial number before the ‘W’ indicating performance at 0°C and the number after showing the lube’s performance at 100°C.

As with engine oils, the higher the number, the greater its viscosity. For motorsport use the ideal is to have low viscosity for low drag and transmissions losses but enough film strength to protect the gearbox or differential. Again, additives will help as covered below. 

Oil Comparative Viscosity Chart

Gear oils are also classed under a GL ( Gear Lubricants ) rating.    

GL-1 to GL-3 = mostly aimed at road use for light load applications. 

 GL-4 – The most common base oil grade, the oils of which contain good volumes of extreme pressure additives. This is typically used for synchromesh gearboxes and transaxles. 

 GL-5 – Containing many more additives than GL-4 oils, GL-5s are used to create gear oils with extreme load resistance, protecting systems such as hypoid gears such a differential. Also used in dog boxes and with suitable additives in limited slip differentials. 

Viscosity Index (VI) 

The viscosity index is a measure of change in viscosity with fluctuations in temperature. In simpler terms, the viscosity index tells you how well a fluid can protect its viscosity during changes in temperature. The higher the number the better the oil is at resisting changes due to temperature. Normally quoted on higher specification oils and helps compare performance. 

The ACEA oil standard

This is the standard of the European Automobile Manufacturers’ Association. It is a quality indicator that is determined by a letter and a number (e.g., A1). The letter indicates the type of engine: 

A – oil designed for petrol engines. 

B – oil designed for diesel engines in private cars. 

 C – for light engines equipped with catalytic converters or particulate filters. 

The number indicates the specific performance the oil must provide. The higher the number, the greater the oil’s performance. The ACEA’s 2016 guidelines define: 

Three category combinations for petrol and diesel engines: A3/B3, A3/B4, A5/B5. 

Five categories for vehicles with a pollution control device: C1, C2, C3, C4, C5. 

The API Oil Standard

Established by the American Petroleum Institute, this standard classifies the product according to dispersive and detergent power, and protection against wear, oxidation, and corrosion. The standard consists of two letters: S for petrol engines, or C for diesel engines. The second letter indicates the oil’s performance. The further along the alphabet the letter, the higher the quality of the oil. For example, an SH oil will have a lower performance than an SN oil.

Oil Types

Mineral Oil

This is refined from a fraction of crude oil, using different techniques including solvent washing and chemical reactions to remove impurities and improve lubrication characteristics. As the oil is derived from crude oil and not changed chemically it is rarely used in motorsport, with a few exceptions. Its main advantage is low cost. Some running in / break in oils are mineral to promote quick bedding of rings etc. 


It is a blend of synthetic oils and mineral oils although there is no industry qualifying quantity of each so the ratio of each can vary considerably. In Motorsport terms a fully synthetic is preferable where the engine value negates any cost savings.   

Fully Synthetic Oil

Although still derived from crude oil, is made by chemical synthesis rather than the refining of crude oil. This is a much more controlled process, meaning that the molecules in the oil are more uniform and exhibit the following benefits: Improved lubricity which gives better protection and reduced wear. Improved viscosity, meaning better and more consistent performance across a wider temperature range. Higher thermal and oxidative resistance, so the oil is less likely to degrade due to high temperature or contamination, meaning fewer deposits and less sludge. It also has reduced volatility and evaporation, resulting in lower oil consumption, especially useful in endurance events. 

Castor Oil

A vegetable-based oil used on some vintage / classic engines, Castor oil has good low-temperature viscosity properties and excellent high-temperature lubrication, but it tends to form gums in a short time and oxidisation properties are poor. Therefore, its usefulness is limited to engines that are regularly rebuilt. Very popular to use in methanol or alcohol fuelled engines It is more tolerant of fuel dilution. Popular as a pre-mix oil in some two stroke engines. Interestingly Castrol took its name from castor oil. If using Castor Oil for the first time it is highly recommended to do after a rebuild as it is does not mix with other oil types.  


It can also be a nightmare trying to figure out which engine additive you need and which is best, continue reading to understand everything from Zinc to a ‘Shockproof’ additive.

ZDDP / Zinc 

Zinc Dialkyldithiophosphate (ZDDP) is an anti-wear additive used in lubricants. ZDDP is required to protect high load contact points such as camshaft lobes and followers. It also acts as an antioxidant to extend oil life under harsh conditions. ZDDP works because it is a polar molecule, so it is attracted to metal surfaces. ZDDP reacts under heat and load to create a sacrificial film that allows ZDDP to protect flat tappet camshafts and other highly loaded engine parts. In effect the ZDDP creates a barrier, so the ZDDP coating wears off in use and not the expensive critical parent metal.  High speed valve trains require a “fast burn” ZDDP that activates quickly.

Driven Racing Motor Oil

However, these “fast burn” ZDDP additives tend to reduce catalytic converter performance. As a result, many oils intended for modern catalyst equipped cars have reduced ZDDP content. While this has been beneficial for reducing emissions the reduced ZDDP content can be very detrimental to classic and racing engines that use uprated cams / valve gear and much higher compression. Cars not requiring a catalyst will benefit from more of this additive, especially if excessive cam or crank wear is an issue. This means some of the high ZDDP content racing oils do not comply with oil standards which are aimed at contemporary cat equipped cars. Look for ZDDP or Zinc % content on the bottle for these high content oils. 

Ester Oils 

An Ester Oil is a synthetic base oil that has been chemically synthesised. The high-performance properties and custom design versatility of esters is ideally suited to problem solving in high performance / racing oils. In plain terms the oil can be engineered for a specific use.  Most top spec oils are a blend of Esters, generally classed as triple ester synthetic oils. 


Nanodrive Engine Oil

Although the surfaces in engines and gearboxes look smooth to the naked eye, microscopic inspection reveals roughness that promote friction and wear, especially in harsh conditions where the oil film can no longer be relied upon for lubrication. Nanoparticles work by adsorbing to the surface of the metal components, both smoothing out the surfaces and providing a physical layer that prevents destructive metal to metal contact – even at high temperatures and pressures. Esters are very valuable functional molecules in a lubricant, providing excellent lubricity as well as high oxidation stability. They are also surface-active, so they efficiently coat the metal surface to protect against friction and wear.

There are different types of esters that interact differently both with themselves and other additives, so the oil blend must be expertly balanced. Millers Oils has developed a unique triple ester formulation that works synergistically to greatly enhance performance. NANODRIVE is the award-winning Millers Oils motorsport brand that incorporates advanced nanoparticle technology for high performance applications, where stress on components is at its limit. Millers put CFS 10w60 NT+ engine oil to an independent dyno test in a Ferrari 250 GT V12 engine; in a back-to-back comparison with a traditional OE-specification oil it achieved peak power and torque increases of 7%, just from an oil change !


Shockproof Gear Oil

Redline Shockproof additives in their gear oils act like a liquid grease, these unique lubricants contain a suspension of solid microscopic particles working as an extreme pressure agent. This offers the “best of both worlds” balance of low drag and superior protection for gear teeth. For example, the Redline heavyweight shockproof oil, gives similar protection to a 75w250 but flows like 75w90 gear oil. Ideal for use in transmissions coping with much more power than intended. 

Friction Modifiers 

Many engine and gear oils contain friction modifiers to reduce friction and in turn wear. Also available as a separate additive to use with limited slip differentials , or as an engine treatment. 

Bike Engined Kit Cars

Cars with motorcycle engines fitted that have a wet clutch ( runs in oil ) will need to use an oil compatible with clutch friction plates. Some oils intended just for car use will cause clutch slip as they are too slippery which reduces clutch plate grip / friction.  

We hope this simple guide helps select an appropriate oil to protect your competition car. If you would like more advice or help choosing, please do not hesitate to contact us by phone or e-mail.  

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