Engine Oil

OIL is essential to lubricate moving parts and carry heat away from the

engine. However, it must be replaced regularly – typically every

10,000-12,000 miles – to ensure it does its job properly.

Every can of oil has two codes on it that need to be taken into

consideration when changing oil. The first relates to the oil viscosity

(how thick it is). This is denoted on a scale in which lower numbers mean

thinner oil and higher numbers thicker oil. The classification was

developed by America’s Society of Automotive Engineers (SAE) and is

recognised worldwide.

It used to be that oils came only in a single viscosity such as SAE 40,

SAE 30 or SAE 20. The lower numbers were for cold weather use. This is

because during cold start-ups the engine needs a thin oil that can

lubricate all the parts as quickly as possible.

Modern oils, however, have multiviscosity, meaning they will serve over

a wider temperature range. The oil is thin enough to lubricate the engine

quickly when it is cold, but does not thin too much when it is hot, thus

maintaining engine protection. Because it adapts to conditions it creates

minimal friction or drag on engine parts, allowing better fuel economy

and slightly more power output. In Britain’s relatively settled climate

the most commonly used multigrades are 5W-20, 5W-30, 10W-30 and 10W-40

(the W stands for winter).

The other code on the can refers to the grade of oil, which reflects

the latest approved manufacturing standards set by the American Petroleum

Institute (API).

The correct code for your car can be obtained from the handbook or the

vehicle manufacturer. For petrol engines the reference is an S followed

by a second letter denoting the time of its approval by the API – the

latest is SM. As a rule it is acceptable to use a more recently approved

oil for an older car but not an older oil for a new car.