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Last week we ascertained that engine oil isn’t the only vital fluid integral to your car’s mechanical systems. While there’s various other fluids helping things work, engine oil is most commonly misunderstood.

In addition to engine oil, other crucial lubricants your car uses include transmission fluid (commonly called gearbox oil) and differential fluid. Transmission and differential fluid play a similar role to engine oil, but they are vastly different products to the oil that goes into your engine.

The most sage piece of wisdom when it comes to engine oil is to stick to whatever the manufacturer has specified for your particular new vehicle. That said there’s still something to be said for understanding what the various numbers and ratings mean.

First things first – the engine oils that are recommended for the overwhelming majority of new vehicles sold in this country are covered by the SAE (Society of Automotive Engineers). As you’d expect, there are American and European rating systems as well and each are subtly different.

The SAE ratings refer to the oil’s viscosity. In short, viscosity refers to how thick (or thin) the oil is at a set temperature. Working out the viscosity reading isn’t anywhere near as difficult as you might think. SAE 30 oil is thinner (which means less viscous) than SAE 40 oil at a set temperature. Pretty simple.

DIY: Engine Oil Explained

Manufacturers spend plenty of time, especially with new vehicles, testing and engineering components to work to various tolerances with a certain specification of lubrication. That’s why it’s usually more sensible to stick to the ratings the manufacturer has noted. Your vehicle’s handbook or owner’s manual will specify the correct oil.

You might have seen the term ‘multi grade’ on the side of engine oil containers and that is the type of oil that most manufacturers recommend for modern engines. Multi grade oils are clever in that they act like both a cold and hot oil rather than being ideal for one or the other.

At start up, multi grade oils can circulate through the engine quickly when it is cold but when the oil heats up it acts like a thicker oil to provide component protection.

Multi grade engine oils will have a reading like 10W-40. That means the viscosity is lower (10) when the engine is cold, but higher (40) when the engine heats up to operating temperature. That way the engine can flow quickly to the areas it needs to get to at start up, then remain protective at operating temperature.

Older cars were designed when there was far less engine oil technology available and as such, many of them are specified to operate with mono grade engine oil. An example of a mono grade engine oil rating would be SAE 30. Despite this, many older cars will work perfectly well with more modern oils, such as the Castrol range we’ve pictured here.

It has to be said that very few modern cars will ever even need a top up of engine oil between services. That’s not the case if you have a classic in the garage. Whether you have any inclination of checking your oil or not, it never hurts to have a basic understanding of what it is and what it’s doing.

CarAdvice DIY is brought to you in partnership with Castrol.

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