What 5W-30 means on engine oil: the numbers explained and which to choose for winter

An oil bottle says 5W-30, 0W-20 or 10W-40, and many people assume those are temperatures. They are viscosity grades under the SAE J300 standard: one number describes how the oil behaves in frost, the other how thick it stays in a hot engine. Here is what each one means and which to choose for a Georgian winter.

W stands for winter

W is winter. The number before the W describes the oil's low-temperature properties, the number after it describes thickness at operating temperature. The two numbers do not depend on each other.

The cold number: 0W, 5W, 10W

Oil is tested in a laboratory at low temperatures: how easily the engine cranks in it and how quickly the oil pump gets it to the top of the engine. The oil takes as its grade the coldest temperature at which it passes the test.

The lower the number, the thinner the oil in frost and the less time the engine runs dry in the first seconds after a cold start. A large share of engine wear happens in exactly those seconds.

The hot number: 20, 30, 40

The second number is the oil's viscosity at 100 °C, in a warmed-up engine. It sets how thick a film stays between metal surfaces under load.

So 5W-30 and 5W-40 behave the same in frost, but in a hot engine 5W-40 is thicker.

Which one to use

The answer is on the oil cap or in the owner's manual. Many modern engines are designed for thin oil, 0W-20 or 5W-30, because variable valve timing and hydraulic lifters are tuned to the factory viscosity. On old or high-mileage engines a thicker grade is sometimes recommended, but "thicker is better" is a myth: oil that is too thick reaches a cold engine later.

Winter in Georgia

In Gudauri and Mestia a January morning can reach -15 or -20 °C. A 10W oil passes its test at -25, so it formally fits, but 5W and 0W oil the engine much faster on such a morning. In Tbilisi and Batumi the difference is smaller, and the factory recommendation is still the limit. We have a separate [article](/en/news/tyre-markings-dot-code) on tyre markings.

Frequently asked questions

Is the 5 in 5W-30 a temperature?

No. It is a viscosity grade that means the oil passes its test at -30 °C. The W marks winter.

Can I use 5W-40 instead of 5W-30?

Both behave the same in frost, but 5W-40 is thicker in a hot engine. If the manual asks for 5W-30, it is better to follow it.

Which is the best winter oil?

The one the manufacturer specifies. If the choice is between several grades, a lower W number is better in a mountain region.

Source: SAE J300 viscosity tables.