There's one measure of viscosity based on SAE standards(Standard Automobile Engineer) and simply another terminology for viscosity in Commercial Grades and Military Grades. The equivalency is as follows:
SAE Com Mil
20 55 1040
30 65 1065
40 80 1080
50 100 1100
60 120 1120
The SAE50 you'd use in your car is equivalent to the straight mineral oil Shell 100 you stick in your aircraft. (or Shell W100 ashless dispersant grade which has various additives).
A multiviscosity grade eg SAE20W50 that you'd probably put in an old classic car is designed for opposed piston engines to be equivalent in terms of viscosity/temp performance to Commercial Grades 65, 80 or 100.