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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.

Vehicle emission control systems can markedly affect the environment within the engine crankcase, and could thereby increase engine deposits, wear, and oil degradation. Tests run using 1965-1970 model United States passenger cars, operating with leaded commercial gasolines in several types of service, evaluated the effects on deposits and wear of three types of experimental vehicle emission control systems: 1. Crankcase storage systems for reducing vehicle evaporative emissions. 2. An exhaust gas recirculation (EGR) system for reducing oxides of nitrogen. 3. Positive crankcase ventilation (PCV) systems for controlling crankcase emissions. In engines operated with production crankcase purging rates, crankcase storage increased engine rusting in short-trip service, and increased sludging and valve train wear in low-speed, stop-and-go service.