The influence of crankcase lubricant viscosity on brake specific fuel consumption (bsfc) has been evaluated in two medium-speed automotive diesel engines. An experimental procedure was established using steady-state operating conditions chosen to increase sensitivity of fuel consumption to changes in lubricant viscosity. The experimental procedure was used to evaluate Newtonian and non-Newtonian lubricants.Results show that, as a group, 15W-40 multigrades are slightly better in fuel economy than a SAE 30 single grade. This difference is attributed to temporary viscosity loss with the multigrades. Use of high shear stress viscosity data brings fuel consumption data with non-Newtonian lubricants into line with that of Newtonian lubricants with the same viscosity and accounts for the difference in fuel consumption observed between two multigrades blended with different molecular weight polymers of the same molecular type.A well known friction reducer shows no effect on bsfc. This confirms the predominance of viscosity in determining fuel economy in this engine.