Engine deposits and wear are greatly affected by engine oil-additive treatment variables and by engine-operating parameters, such as oil-change interval and oil filtration. While each of these two major elements has been investigated extensively, little is known about interactions between these elements. Tests with 1963-1967 model United States passenger cars, operating with leaded commercial gasolines in several types of service, evaluated effects on deposits and wear of: 1. Ashless (nonmetallic) dispersants. 2. Zinc dialkyldithiophosphate (ZDP) type and concentration. 3. Interactions between dispersant and ZDP. 4. Interaction among dispersant concentration, oil-change interval, and supplementary bypass oil filtration. Sludge and varnish deposit control differed widely among three dispersants used at equal concentrations. Increasing the concentration of the best dispersant reduced sludge but not varnish.Increasing ZDP concentration above about 0.06 weight % zinc greatly reduced valve train scuffing. At one zinc concentration (0.12 weight %), valve lifters wore similarly using ZDP made with either primary or secondary alcohols.Increasing ZDP concentration decreased cam and lifter wear with high-dispersant concentrations, but increased wear at low-dispersant concentrations. Less wear occurred with low ZDP concentrations in low-dispersancy oils than at any ZDP concentration in high-dispersancy oils.Deposits and wear increased with increasing oil-change interval, and at each interval, more deposits and generally more wear occurred using a low-dispersancy oil than a high-dispersancy oil. With high-dispersancy oils, supplementary filtration had no effect on deposits and wear. With low-dispersancy oils, supplementary filtration reduced sludging and cam and lifter wear, but did not affect varnish or piston ring and bearing wear.