Viscosity explained
What the two numbers in a grade like 5W-30 mean, and why they matter at a cold start and at full operating temperature.
Grade decoder
Two numbers. Two temperatures.
Pick a grade. The first number is about a cold start, the second about an engine at full heat.
−30 °C
The first number: cold
5W means the engine still cranks on this oil at −30 °C: the lower the number, the colder the start it copes with.
9.3 to 12.5 cSt
The second number: hot
30 means the oil is 9.3 to 12.5 cSt thick at 100 °C, the heat of a running engine: the higher the number, the thicker the film that carries the load.
Single grades such as SAE 40 have only the hot number. They carry no W rating, so they are not rated for cold starts.
Values from SAE J300, the standard that sets engine oil viscosity grades. cSt (centistokes) measures how easily oil flows: the higher, the thicker.
Drop test
Thin oil moves first.
The same steel ball in four grades. Thicker oil holds it back, and a cold start makes every oil thicker.
- 0W-200.00 s
- 10W-300.00 s
- 15W-400.00 s
- 20W-500.00 s
Illustrative, not lab data.