The effects on connecting rod bearing wear of several engine oil compositional variables have been studied. A single-cylinder CLR engine installed at Automotive Research Laboratories, Inc., in Chicago has been used to study the effects of viscosity on connecting rod bearing wear for Newtonian lubricants and for a series of non-Newtonian multigrade oils on which previous field test experience was available. The results from this series of experiments showed that in the particular engine and under the conditions used, viscosity had a relatively minor effect on connecting rod bearing wear with the Newtonian oils. With the non-Newtonian multigrade oils the detergent inhibitor package showed up as a major variable and it was not possible to segregate the effects of the viscosity improvers used to prepare four test oils from the effects of the detergent system used. Further work showed that the detergent inhibitor effects could be drastically altered by substitution of individual additive components in the test oil. In a second series of data the authors have carefully reviewed connecting rod bearing wear data from two field tests; one reported in the literature (3) and a second test run by the authors' laboratories. Extensive data analysis was done to correlate connecting rod bearing wear (and other engine wear) with normal lube oil compositional variables such as VI shear stability, used oil acidity and used oil insolubles. The best correlation was found between connecting rod bearing wear and pentane insolubles in the first drain of the test. Both these sets of tests indicate an abrasive mechanism as the most likely cause of steady, non-castastrophic connecting rod bearing wear.