A series of polymer-containing fluids was evaluated in a modified ASTM D 2882 pump loop to determine both permanent viscosity loss and flow rate as a function of test time. Flow rate did not change over the course of the test. It correlated equally well with either new oil high-shear-rate viscosity or used oil kinematic viscosity.The current ISO viscosity classification system provides no means to differentiate the levels of performance observed. It is proposed that a new, more meaningful viscosity classification system be adopted for hydraulic fluids. This system should maintain continuity with the current system, but should account for permanent viscosity loss of polymer-containing fluids. Measured low temperature properties should also be part of any new viscosity classification.