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The lubrication properties of engine oils change in the early stage of car running and after that the oils keep a good antiwear property and a low friction coefficient even after 10,000 km running. On the other hand, the thin layer chromatography spot characteristic of ZnDTP disappeared after 2,000 km running. The reason why the antiwear property of the used engine oils was retained after decomposition of ZnDTP has been discussed. Analytical results indicated that P=S bond of the ZnDTP disappeared at a running distance of 2,000 km but P-O-C bond remained after 10,000 km running. In laboratory tests, ZnDTP decomposed by heating at a temperature of 120 °C with eliminating olefins after an induction period of 100 h. With a sample oil containing ZnDTP alone, the additive decomposed into precipitate upon heating to 135 °C for 60 h, and the antiwear property of the oil deteriorated.