Relative oil-film thickness has been measured in a main bearing of a fired, single-cylinder CLR engine. The bearing is electrically insulated from the block and a small voltage impressed between the bearing and crankshaft. When film thickness becomes less than a critical value, a series of electrical discharges occurs. The relative film thickness is determined by the extent of the electrical discharge.Relative film thickness was found to increase with speed and viscosity and decrease with load as predicted from the Sommerfeld parameter. Oils containing V.I. improvers gave thinner oil films than predicted by their low-shear viscosity, but slightly thicker films than predicted by their high-shear viscosity. One oil, containing a low molecular weight polymer, was even better than its low-shear viscosity Newtonian counterpart.