Originally Posted by alfafamily
Thats interesting ,so why does the the figure vary anything from 0 to 20? Are you saying that an older type diesel engine that asks for a 15/40 is better off with a 10/40?
THe 0-20 are arbitrary numbers, assigned to an oil after testing to see which norm it meets. A 10w-40 flows better at low temperatures than a 15w-40, a 5w-40 flows better still... 0w-40 is essentially the same as 5w-40, only the pour point is lower. All these oils will flow roughly the same at operating temps, but the "thinner when cold will start protecting the engine much earlier"
so yes, a 10w-40 is superior to a 15w-40. Don't forget that the viscosity and quality of oils mantioned in the user manual is the minimum required. That doesn't mean you can't do better.