Quote Originally Posted by aaronng
2 steps down for the 2nd number
1st number is SAE oil viscosity equivalent at -18ºC, while 2nd number is SAE equivalent at 100ºC.
The first number 10w means it has a viscosity like an SAE10 oil at -18ºC. 5w means SAE5. 0w means it is the same as 5w, but it can be poured at a lower temperature (10ºC lower than 5w before becoming not liquid enough to flow).

So 1 step down from 10w is 5w, while 0w is the same as 5w but has better flow capacity at lower temperature.

At the upper end it is like normal for 100ºC. 30 means equivalent to SAE30, and so on. I wouldn't go even 1 step down, as that means the oil (20) is very thin. Going up 1 step is ok. Going 2 steps up is not as good, because it is too viscous and cannot get in between the 2 metal surfaces in small gaps in a fine tolerance engine.
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