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Zinc dialkyldithiophosphates (ZDDPs) provide efficient and effective anti-wear, anti-oxidation and anti-corrosion properties in engine oil formulations. Two kinds of formulated engine oils were investigated including Oil 1 containing secondary ZDDPs, and Oil 2 containing primary/secondary ZDDPs. It revealed that different types of ZDDPs exhibits different volatilities and have different effects in the Sequence IIIG test. ZDDPs with higher molecular weight alcohols exhibit a relatively low volatility. The combination of primary/secondary ZDDPs exhibits higher phosphorus volatility in the Sequence IIIG, but have the same anti-wear protective behavior than that of the secondary ZDDPs in field tests. The field test results of phosphorus volatility and viscosity increase show encouraging agreement with the results of the Sequence IIIG test. However, the field test exhibits lower phosphorus volatility than the Sequence IIIG engine test.