A fired multi-cylinder laboratory dynamometer engine test has been developed for measuring the effect of engine oil on fuel consumption with high statistical confidence. The range of test conditions employed encompasses both the hydrodynamic and boundary lubrication regimes so that frictional, as well as viscometric effects can be detected. The effects of viscosity and SAE viscosity grade for Newtonian and non-Newtonian test lubricants, and the effect of viscosity index improver molecular weight within an SAE grade have been studied. The results obtained are largely explained by the rheological properties of the test fluids. The relationship between the performance of non-Newtonian and Newtonian oils has been found to be dependent on engine type, presumably due to engineering characteristics which affect the shear stresses and temperatures imposed upon the lubricant.