Engine oil viscosity loss was investigated in highway service and in ASTM D-3945 Procedure A (diesel injector) shear stability test. The six oils involved contained various viscosity index improvers and had D-3945 100°C kinematic viscosity losses ranging from low to high. The losses in service were highest for an oil with one of the lowest D-3945A viscosity losses. D-3945A also did not correlate with service using 150°C high shear viscosity losses. D-3945A shearing severity was modified over a wide range, but it still did not rank oils like highway service. Results indicate a need for a more meaningful viscosity-loss bench test.