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).
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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.