What Oil Do You Use for Diesel Engines?

[Lister]

Not to be argumentative, but the xW has very precise specs, and it must meet those specs to get the cold pumping xW on it...and most of those are well below zero C.

The 40C viscosity doesn't have anything to do with the xW spec, particularly as most people in the world never have a 40C Cold Start.

Just so we are clear, The "W" Stands for "weatherly"(name of a guy) not winter. Its an American Petroleum Institute (API) standard. The Lower number in your multigrade viscosity index indicates the oil will be "NO THINNER"(in other words, always thicker than) than 15 when its COLD in a 15w-40 (Knowing that oil thins as it gets warm and thickens when it cools). The higher number means it will be no "thicker" than 40 when its HOT. The general understanding is usually that the lower number is cold start, this incorrect, if it were, then a 0w-50 Full Esther based synthetic oil would provide ZERO lubrication or atleast as much as water and we all know this aint right.

Information here has been taken from Valvoline and Pennzoil training ....
Cheers Glenn