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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.

To meet exhaust emission standards, nearly all 1975 model U. S. passenger cars use catalytic converters in conjunction with unleaded gasoline. While it has been established that lead and phosphorus from gasoline are deleterious to catalyst performance, much less is known about any similar effect of elements normally present in conventional engine oils. In addition, the ability to protect engines from excessive deposits and wear is essentially unproved for engine oils in which the phosphorus and metals contents have been either reduced (low ash oils) or eliminated (ashless oils). To obtain catalyst and engine performance information on such oils, tests were run using 95, 1972-1973 model passenger cars, operated with unleaded gasoline in several types of service. Forty cars were equipped with 1975 production-prototype underfloor catalytic converters containing pelleted oxidation catalysts.