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.
Neither exhaust emissions nor catalyst conversion efficiency were affected by using either a low ash or an ashless oil instead of a conventional oil. However, oil consumption was less than typical for such service, so the effect of oil additives on catalyst durability may have been proportionately less.
Five experimental ashless oils were all deficient in engine performance in varying degrees, compared to conventional commercial SE-quality oils, with respect to cam and lifter wear, oil ring sticking, and piston varnish deposits.
It is concluded that reduction in phosphorus or metals content of current, conventional, commercial oils does not appear to be necessary to meet current exhaust emission standards with current General Motors (GM) catalyst systems. However, development of ashless oil technology should continue in the event such oils are necessary to meet future, more stringent engine or emission requirements.