The temporary viscosity loss experienced by non-Newtonian lubricants, operating in a simulated engine bearing, has been determined. Included in the study were; lubricant blends containing 13 commonly-used viscosity index improvers and six commercial multigraded engine oils. Bearing friction and minimum oil-film thickness data were used to determine the temporary viscosity loss. For the 13 viscosity index improvers, this temporary loss ranged from 20-96% of the polymer contributed viscosity. Results indicate that low shear rate viscosity determinations, such as the currently used ASTM D445, should not be used to characterize the behavior of these lubricants in high-speed engine bearings.