From our insanely detailed guide:

Engine Oil

Any liquid can be used as a lubricant - just look at water slides. These operate on the same principle as lubrication inside the engine: A constant flow of water separates our bottoms from the surface of the slide and reduces friction. If the flow stops, our weight soon forces the water out from that gap, and we are stuck wiggling down the tube. Oil is a much better lubricant (for engines, at least) - it keeps an unbroken film under pressure, it won’t evaporate and doesn’t cause parts to rust.

What engine oil does

To see why the oil is so important, let’s first look at its main functions.

Reducing friction

When two parts are in contact, and move alongside each other, they rub together. This rubbing, or resistance to movement, is called friction . The energy that is used in overcoming friction is converted into heat, and not movement. You can feel this when you rub your hands together quickly - heat builds up and your hands are warmer. This is useful for a person on a ice-cold day, but in an engine all the energy that is converted to heat is energy that isn’t being used to produce movement.

The amount of friction between parts depends on:

  • The materials that the parts are made from - rubber has more friction than chrome.
  • The surface finish . Smooth, polished surfaces slide more easily past each other. That’s why bearing surfaces are machined to a high gloss.
  • And the load pressing those surfaces together . In the engine this means that the main bearings, in which the crankshaft rotates, are subject to the highest forces - they take the weight of the heavy crankshaft and the piston assemblies.
  • Movement between two surfaces also creates wear, as microscopic irregularities in the surfaces bang into each other and chip off.

    The main role of lubricant in the engine is to keep moving surfaces separated. With the metal surfaces separated, there is no contact between and therefore no friction or wear.

Cooling the engine

Lubricant also absorbs heat on its travels around the engine. It carries that heat back to the oil pan, where it dissipates the heat into the surrounding air. Some oil pans with have cooling fins to maximise the oil cooling that occurs.

In higher performance engines the oil pan doesn’t provide enough cooling for the oil, so a oil cooler will be added. This is usually a small radiator fitted near to the coolant radiator, through which air passes and cools the oil.


Oils pick up any contaminants in the engine and carry them away. Larger particles are filtered out into the oil filter, and small, soft particles remain suspended in the engine oil until an oil change. Detergent chemicals are added to the oil when it’s refined so that particles remain suspended and aren’t deposited onto surfaces.

A coating of oil prevents metal parts from contacting oxygen and corroding. Preventing corrosion is a nice added-benefit of the lubrication system. Even the oldest, rustiest car will usually have shiny engine internals - so long as there’s been a supply of oil circulating once in a while.

Types of oil

Oil grades explained

Viscosity is a measure a liquid’s resistance to flow - in other words, how thick the liquid is. A thick liquid, like bread dough, has a high viscosity, and a thin, runny liquid like water, has a low viscosity.

When moving parts are separated by oil - the only resistance to the moment of those parts is the viscosity of the oil. So we’d like the viscosity as low as possible to keep the engine efficient. However, the viscosity also affects the rate at which oil is squeezed out between two surfaces. If the oil has a very low viscosity, it will flow out of the clearance too quickly and the parts will make contact. It’s clear then, that we need a balance.

Engine manufacturers specify the viscosity of oil that is needed based on a number of criteria - the amount of oil, operating temperature, maintenance requirements, efficiency, and - probably most importantly - the bearing clearances. Engines with very tight bearing clearances are able to handle a thinner engine oil.

For oil, its viscosity is specified as a grade set by the Society of Automobile Engineers (SAE). An grade of SAE 50 would be a viscous oil, while a grade of SAE 10 is a thin oil. As engines have become more efficient they have trended towards thinner oils.

There’s a complicating factor: oil becomes thinner as it gets hot. If you’ve changed oil in an engine then you’ll know that it flows far more easily when warm than cold. This means oil that was perfect for an engine in summer may be too thick for winter, so historically engines would have used one grade of oil in winter, and one in summer.

Oil manufacturers developed multigrade oils to deal with this situation. By including a chemical additive, the effect of temperature on oils can be reduced and a single oil is now used year-round in all engines.

Multigrade oils are rated with two numbers, for example a 10W40 oil has an SAE grade of 10 in winter (signified by W) and an SAE of 40 in normal conditions. Manufacturers will provide a table indicating the optimum grade of oil to use based on the climate of operation for a vehicle

[Engine oil grade table]

Mineral vs Synthetic oils

While there are animal, vegetable and mineral oils, in engines we only use mineral oils. Mineral oils are refined from crude petroleum which is extracted from the ground.

Over the past two decades or so, synthetic motor oils have been developed which can provide benefits for high performance engines. Surprisingly, synthetic oils are also generally produced from mineral oils - using only the purest parts, to which are added chemicals to provide precisely engineered chemical properties and viscosities.

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