As I remember, multi-grade oils are measured as follows...in the case of say, 20W-40. The first viscosity is measured at 0 F which is denoted by the "W" for "winter". The next viscosity is measured at 210 F. The oil doesn't become thicker, but because of it's high viscosity index, it measures as a SAE 40 at 210 F.( it doesn't thin out as much as a low VI oil at the same temp). This is a slippery subject.
Stuart.
