When we measure the viscosity of oils at 100C (xW20, xW30, xW40 , etc) they all are generally within the published ranges for 20 , 30 , 40 weight oils.
Why then, does a 5W viscosity (at 40C per ASTM D-445) vary so wildly, depending on the operating viscosity.
For instance, I looked up some RLI...