Engine oil viscosity: what 0W-20, 5W-30 and 5W-40 mean
The numbers on the bottle are a grade under the SAE J300 standard, not marketing. They describe how the oil behaves on a cold start and in a warm engine. There is no need to guess: the right answer is almost always in your car’s owner’s manual.
The first number
W stands for winter. The number in front of it tells you how cold it can get while the starter can still turn the engine and the oil pump can still push oil to the bearings. The lower the number, the lower that temperature.
| Grade | Engine cranking | Oil pumping |
|---|---|---|
| 0W | down to −35 °C | down to −40 °C |
| 5W | down to −30 °C | down to −35 °C |
| 10W | down to −25 °C | down to −30 °C |
| 15W | down to −20 °C | down to −25 °C |
Tashkent rarely gets colder than −10 to −15 °C, so 0W versus 5W makes little difference to whether the car starts. It matters elsewhere: 0W oil reaches the moving parts faster in the first seconds after a start, which is when most engine wear happens.
The second number
The number after the W is the viscosity of warm oil. For each grade the standard sets a viscosity range at 100 °C and a minimum HTHS viscosity at 150 °C and high shear, the conditions inside crankshaft bearings and between piston and cylinder.
| Grade | Viscosity at 100 °C, mm²/s | HTHS at least, mPa·s |
|---|---|---|
| 20 | 6.9–9.3 | 2.6 |
| 30 | 9.3–12.5 | 2.9 |
| 40 | 12.5–16.3 | 3.5 |
| 50 | 16.3–21.9 | 3.7 |
The key point: 0W-20 and 5W-20 are the same once the engine is warm; 5W-30 and 5W-40 are not. The second number sets the thickness of the oil film under load, and engines are designed around a specific grade.
Why you don’t need “thicker oil for summer”
An engine with a working thermostat runs at the same 90–105 °C in January and in July. Hot weather barely changes the oil temperature unless the car idles for hours with the air conditioning on or tows a trailer. “Grade 40 in summer, 30 in winter” dates from the carburettor era.
Oil that is too thick does harm too. Modern engines with narrow oil passages, variable valve timing and timing chains are built for a specific oil pressure. In an engine designed for 0W-20, a 5W-40 reaches the parts more slowly after a start and can upset the valve-timing actuators. The opposite mistake, a grade 20 where 40 is required, leaves too thin a film under load.
How to choose the viscosity for your car
- Open the owner’s manual. The maintenance section lists the permitted SAE grades, sometimes by outside temperature.
- Check the specification. Viscosity is only half of the requirement; you also need a specification such as API SP, ACEA C3 or GM dexos1 Gen 3.
- If several grades are allowed, use the first one recommended. The thicker permitted grade makes sense for a worn engine that has started to use oil.
- No manual? Look at the sticker under the bonnet or the oil filler cap, or send us the model, year and engine size and we will help.
Can you mix oils?
On the road, topping up with a different brand of the same grade and specification is fine: certified oils are compatible. At the next opportunity, change the oil completely to get back the properties the engine is designed for.
Not sure which oil fits your car? Send us the model, year and engine size.
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