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Zinc dialkyldithiophosphate (ZDDP) is the most common anti-wear additive used in engine oils and in order to prevent wear,ZDDP is known to form glassy polyphosphate films.These films cause friction in the mixed lubrication regime to increase which is detrimental to the fuel economy performance of engine oils.Most investigations of the form ation of ZDDP tribofilms and the properties of these films have concentrated on tribofilm formation from unaged oils.Anti-wear additives must work throughout the engine oil drain interval.Furthermore,ZDDPs synthesized with different alcohol groups decompose at different rates.Therefore,the effect of oil aging on the tribofilm formation and tribofilm properties of engine oils formulated with different ZDDPs was examined.Furthermore,the engine oils which were examined were aged in engine tests in order to best simulate the degradation pathways which would be encountered by the oils in actual operation.