The role of non-Newtonian fluid characteristics in lubricant performance was studied in a variety of V.I. improved lubricants. It was shown that proper choice of V.I. improver can yield a lubricant which undergoes both small temporary and permanent viscosity losses, thus giving nearly Newtonian behavior but with high and low temperature performance advantages over straight mineral oils. In hydraulic fluids, it was shown that different V.I. improvers resulted in varying amounts of internal leakage in both vane and internal gear pumps. In the case of vane pumps, the favorable Newtonian characteristics of shear-stable V.I. improvers assured outstanding pumping performance. It is believed that control of transmission leak-down was primarily responsible for the favorable influence a shear-stable V.I. improver had on the maximum transmission sump temperature at which the wide-open throttle 2-3 upshift will take place. Finally, the shear stability of the V.I. improver in engine oils was shown to influence journal wear protection provided by that oil.