Rheological studies of an SAE 10W-40 motor oil were made over temperature and shear rate ranges typical of service. These include evaluations of both new and used oils. Determinations were made of the permanent viscosity changes during service, as well as the temporary non-Newtonian changes with shear. Data were taken at 60, 100, 210, and 250 F, which evaluated the oil from the initial first Newtonian to the second Newtonian regions. New oil viscosity measurements have little relation to the characteristics exhibited over most of the operating period between drains in an engine. Oils having similar initial viscosities can differ in service because of differences in formulation or service severity. It is evident that evaluation of the relatively “stable” used oil near or at the second Newtonian high shear rate region is the most informative. No convenient technique presently exists for such measurements on a large volume basis.