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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.

Lengthening the engine oil-change interval would reduce the frequency of automotive maintenance, reduce the amount of oil required to service the car population, and reduce the potential pollution problem resulting from the disposal of the used oil. Tests were run using 1967-1972 model U.S. passenger cars, operating in several types of service. Using unleaded instead of leaded gasoline reduced deposits and wear. With unleaded gasoline, doubling the oil-change interval had no significant effect on deposits and wear, but did increase oil filter plugging frequency. Deposits and wear were less with unleaded gasoline and doubled oil-change interval than with leaded gasoline and “standard” oil-change interval. Doubling the concentration of additives used in current quality oils was more effective at reducing deposits and wear than was doubling the engine crankcase ventilation rate.

Forty-one short-trip service tests were run using 1964-1969 model U.S. passenger cars and 19 commercial or experimental oils evaluating several aspects of engine rusting. These factors included differences in rusting severity among engine models, effects of fuel variables on rusting, and the effects of oil additive type and concentration on rusting. Rusting severity varied widely among engine models, but different engines ranked similarly the rust protection provided by oils with differing additive treatments. Use of either nonleaded gasoline or LPG instead of leaded gasoline greatly reduced engine rusting. At equivalent additive treatment levels, several magnesium sulfonates provided better rust protection than did certain calcium sulfonates, calcium phenates, or a barium sulfonate. Rust protection increased greatly and approximately linearly with increasing magnesium content.

The recent increase in excessive camshaft and lifter wear after extended service has shown that some SE-quality engine oils do not provide adequate protection. To determine the effects of oil additives on wear, controlled tests were run using 1972-1974 model cars, unleaded gasoline, and either SE commercial products or experimental formulations. Field experience with 1970-1975 model trucks, leaded gasoline, and SE/CC or SE/CD oils was also investigated. With some commercial oils, in both controlled tests and field experience, excessive wear sometimes occurred after extended service, even with recommended oil-change intervals. Generally, protection from excessive wear was best provided by those oils containing pre-dominantly alkyl ZDP (zinc dithiophosphate) antiwear additive instead of aryl ZDP. These results show that a laboratory engine test is needed to evaluate the long-term wear protection of engine oils.