This is a progress report of a continuing program aimed at understanding the role of the engine oil viscosity index improver (VII) in ORI. In this study two polymers, one hydrocarbon, one ester, have been examined in a closely controlled Octane Requirement Increase (ORI) experiment. A variety of measures were employed to block out other variables. The two test oils were separately evaluated in two similar 1975 cars operating with commercial unleaded gasoline. A detailed analysis of the deposits resulting from 24,000 km (15,000 miles) of dynamometer operation supplemented the conventional octane ratings. Conclusive differences were not found in terms of observed ORI or in the total volume of combustion chamber deposits. However, the olefin copolymer VII was directionally lower in ORI and seemed to have a qualitative advantage in engine deposit formation and location. Several of the experimental techniques used in the current work appear to have potential for future study.