The effects of engine oil viscosity and chemical composition on connecting rod bearing wear have been studied using single cylinder CLR engine tests. Two procedures were followed. One procedure, developed by Automotive Research Laboratories, Inc. (ALI), was used to evaluate both single grade, Newtonian oils and multigrade, non-Newtonian oils. The results indicated that there was no relationship between the high temperature, high shear rate viscosity of multigrade oils and bearing demerits. The chemical composition of the engine lubricants appeared as a primary variable which overshadowed the viscometric considerations.A more severe test procedure was developed using the single cylinder CLR engine. The results from this procedure were found to correlate with those from the ALI test programs. Chemical composition was again found to have a dominant effect on bearing wear. It was possible to reduce the bearing protection provided by an SAE 30 oil to less than that of an SAE 5W oil by the selective substitution or elimination of the additive components.Additive components, or materials derived from them, were found to be present on the inner surfaces of the used bearings. Such films may affect wear when the bearings are subjected to mixed and boundary lubrication.