Engine oil grades explained

Take 5W-30 as an example. The first number with the W (for winter) shows how easily the oil flows when cold, and the second number shows how thick it stays at normal running temperature. The lower the first number, the better it flows on a cold start.

That is only half the story, though. A bottle also carries approval standards that matter just as much as the grade, and the right choice for your engine depends on the vehicle. Here is how to read the label and find the correct oil, from our workshop at Longcombe Cross near Totnes.

What do the numbers on engine oil mean?

Oil grades follow a system set by the Society of Automotive Engineers, known as SAE. A multigrade oil such as 10W-40 behaves like a thinner oil when it is cold and like a thicker oil when it is hot. That is why one oil can work through a cold morning and a long motorway run.

The W figure relates to cold flow: 0W flows more readily than 10W in low temperatures, which helps oil reach moving parts quickly at start-up. The second figure relates to thickness at operating temperature. A higher number does not mean better protection. It means thicker oil, which may not suit an engine designed for a thinner one.

How do I find the right oil for my car?

The handbook is the reference. It lists the viscosity grade and the approval specification your engine needs. Some cars also show the oil specification on a sticker under the bonnet or on the filler cap, but do not rely on the cap alone. Do not assume a neighbour's oil suits your car because the models look alike, since engines within one range can differ.

If you cannot find your handbook, a garage can look up the correct grade for your registration. When you ask Automaster for a quote for a service, tell us the make, model and year, and we will work from that.

• Viscosity grade, such as 5W-30
• Approval or specification listed in the handbook
• Whether your engine is petrol or diesel
• Any manufacturer note about long intervals
• Quantity the engine takes, including the filter

What are ACEA and API oil specifications?

Alongside the grade, you will see standards such as ACEA or API on the bottle. These show the oil has passed tests set by European or American industry bodies. Manufacturers often add their own approvals, and many engines require oil that carries a particular one.

Two oils with the same grade can behave very differently if their specifications differ. Using oil that meets the stated approval matters because it relates to wear protection, cleanliness and compatibility with exhaust components.

Does diesel need a different oil from petrol?

Often yes, but it depends on the vehicle. Many modern diesels with a diesel particulate filter need a low-ash oil, sometimes described as low SAPS, so the filter does not clog with oil residues. Some petrol engines also call for specific oils, particularly turbocharged or direct-injection types.

Because of this, a general-purpose oil from a shelf may not be suitable. Check the specification in the handbook, not only the grade.

What happens if I use the wrong oil grade?

One top-up of a slightly different oil in an emergency is unlikely to cause immediate harm, but regular use of the wrong grade or specification can affect cold-start protection, oil consumption and fuel use, and may harm a particulate filter. It depends on the engine.

If the oil warning light comes on while driving, stop safely and switch off. Do not continue to drive on low oil pressure. Check the level when the engine has cooled and the car is on level ground, and get it inspected if the light stays on or the level is low.