What the numbers on engine oil mean

Advice23 Oct 2026

Duckhams engine oils on a yellow drum in front of a car with its bonnet open, including a 20W-50, a 5W-30 and a bottle of brake fluid

The numbers on engine oil, such as 5W-30, are its viscosity grade: how easily it flows. The first number, with W for winter, is how it flows when the engine is cold. The second is how thick it stays once the engine is hot.

Your car's manufacturer lists the grade, or grades, your engine is built for. That is the one to use.

What does the W in 5W-30 stand for?

Winter. The number before the W rates how the oil flows at low temperatures. The lower it is, the faster the oil gets round the engine after a cold start, before everything has warmed up. A 0W oil flows more easily from cold than a 5W, and a 5W more easily than a 10W.

What does the second number mean?

How thick the oil stays at normal running temperature. It is measured at 100°C. The higher the number, the thicker the oil: a 40 is thicker when hot than a 30, and a 30 thicker than a 20.

Is 5W-30 or 5W-40 better?

Neither is better on its own. They flow the same from cold, and 5W-40 is thicker once the engine is hot. What matters is which one your engine was designed for. Some cars list more than one grade, often for different climates, and any listed grade is fine to use.

Can I use a different grade from the one in my handbook?

Only if your manufacturer lists it. A grade that isn't listed can change how quickly oil reaches the engine at start-up and how well it protects at running temperature, and it may affect your warranty.

Why do newer cars use thinner oils like 0W-20?

Thinner oil takes less effort to pump round the engine, which helps fuel economy and emissions, and many recent engines are designed for it. A thicker oil in one of these engines can lose that benefit. A thin oil in an older engine built for a thicker one may not protect it properly. In both cases, the grade in your handbook is the right one.

What is a single-grade oil, like SAE 30?

An oil with only one number is a single-grade, or monograde, oil. It has no winter rating, so it thickens more in the cold than a multigrade like 5W-30. Some vintage engines were designed for one; modern engines need a multigrade. Our classic car oil guide explains more.

Is the grade all I need to check?

No. Your handbook also gives a specification, such as ACEA C3 or VW 504 00, which sets how the oil must perform. Two 5W-30 oils can meet different specifications, so match both the grade and the specification. Find My Oil shows both from your registration, with the Duckhams oils that meet them.

Duckhams oils by grade

Questions

Is 0W-20 thinner than 5W-30?

Yes. It flows more easily from cold, because 0 is lower than 5, and it is thinner when hot, because 20 is lower than 30.

Does a higher number mean better protection?

No. It means a thicker oil when hot. The best protection comes from the grade and specification your engine was designed for.

Where do I find my car's oil grade?

In the handbook, usually in the section on servicing or technical data. Or enter your reg in Find My Oil.