Engine Oil Viscosity Explained: What Do 0W-20, 5W-30 and 10W-40 Mean?
The numbers on an oil bottle, such as 5W-30, are its SAE viscosity grade, defined in the SAE J300 standard. They tell you how easily the oil flows when the engine is cold and how thick it stays once the engine is hot. Using the grade your manufacturer specifies matters, because modern engines are designed around a particular oil thickness.
How to read a viscosity grade
Take 5W-30 as an example:
- 5W is the winter rating. The lower the number before the W, the more easily the oil flows at low temperatures, so it reaches moving parts faster on a cold start. A 0W oil flows more easily in the cold than a 5W, and a 5W more easily than a 10W.
- 30 is the hot rating, based on the oil's viscosity at 100°C. A higher number means a thicker oil at running temperature, so a 40 oil is thicker than a 30 when hot.
Common grades and where they are used
| Grade | Typical use |
|---|---|
| 0W-20 | Modern fuel-efficient petrol and hybrid engines, including many Toyota and Volvo engines and newer VW Group engines that need VW 508 00 / 509 00. |
| 0W-30 | Modern petrol and diesel engines designed for a thin, fuel-saving oil, including many Volvo, Peugeot, Citroën and Ford EcoBlue engines. |
| 5W-20 | Many Ford EcoBoost petrol engines, which call for Ford WSS-M2C948-B. |
| 5W-30 | The most common grade for modern cars, used by VW Group, BMW, Mercedes-Benz, Ford, Vauxhall, Renault and others, usually with a specific approval. |
| 5W-40 | Many petrol engines and older diesels, including VW pump-injector (PD) TDI engines that need VW 505 01. |
| 10W-40 | Older petrol and diesel engines, often with a semi-synthetic oil. |
| 15W-40 | Older engines and heavy-duty diesels, usually with a mineral oil. |
| 20W-50 | Classic cars and some older high-mileage engines. |
| 10W-60 | High-performance engines, such as some BMW M models and sports cars. |
Can I use a thicker or thinner oil than recommended?
Use a grade your handbook lists. Modern engines have tight tolerances, variable valve timing and hydraulic chain tensioners that depend on the specified oil thickness. A thicker oil can slow oil flow on cold starts and increase fuel use, while a thinner one may not hold a strong enough film at high temperature. Some handbooks list several grades for different climates, and any of those is fine.
5W-30 or 5W-40: which should I use?
Both flow equally well when cold, and 5W-40 is slightly thicker when hot. In practice the approval decides it. VW 504 00 / 507 00 oils, for example, are usually 5W-30, while VW 505 01 oils for older PD diesels are usually 5W-40. Choose the grade that comes with the approval your engine needs.
Does viscosity tell me everything?
No. Viscosity only describes thickness. The additives that clean the engine, prevent wear and protect a diesel particulate filter are covered by specifications such as ACEA categories and by manufacturer approvals. Always check both. Our guide to choosing the right engine oil walks through the whole process.
Frequently asked questions
What does the W stand for in 5W-30?
Winter. The number before the W rates how the oil performs at low temperatures, not its thickness at running temperature.
Is 0W-20 thinner than 5W-30?
Yes. A 0W-20 flows more easily when cold and is thinner at running temperature. Only use it in engines designed for it.
Should I use a thicker oil in a high-mileage engine?
Only if your manufacturer lists that grade for your engine. A thicker oil will not repair worn parts, so if an older engine is using a lot of oil, have it checked.
Is 0W-30 better than 5W-30?
A 0W-30 flows better on very cold starts, and both are the same thickness when hot. Either is fine where your handbook lists it, as long as the oil also carries the required approval.