What the numbers on an oil bottle actually mean

Every bottle of engine oil carries a grade like 5W-30. It is the most visible thing on the label, and the most misunderstood.

The two numbers

The first number with the W (for winter) describes how the oil flows when cold — the lower it is, the more easily the oil moves round a cold engine at start-up, which is when most wear happens. The second number describes how it behaves at operating temperature.

So a 0W-30 and a 5W-30 protect the same way once warm; the 0W flows better on a January morning.

Why the grade is not enough

Two 5W-30 oils can be built completely differently. What separates them is the manufacturer approval printed underneath — BMW Longlife-04, VW 504 00, MB 229.51 and so on. Those approvals cover the chemistry your engine needs: how the oil deals with soot, whether it will block a particulate filter, how long it lasts.

Putting the right viscosity in with the wrong approval is one of the more expensive mistakes you can make in a modern engine.

How to get it right

Check the handbook, or enter your registration on our vehicle finder. We match on the approval your engine requires, not on the grade alone — so what you are shown is oil that is actually approved for your car.