Zinc dialkyldithiophosphate (ZnDTP) is a well-known multi-functional additive for lubricating oils that has superior oxidation inhibition and antiwear performance. All the engine oils for four-stroke automotive engines contain ZnDTP because no additive has been found to have better performance. However, since it was pointed out that ZnDTP poisons the exhaust catalyst, reducing ZnDTP concentrations in engine oil without losing the superior antiwear performance has become an important priority. The friction and wear characteristics of intentionally deteriorated ZnDTP, which can simulate used engine-oil, were studied using a four-ball tribometer. Both sec-C6 and prim-C8 ZnDTP were degraded by reacting with cumenehydroperoxide (CHP), which modeled the hydroperoxide present in combustion gas. Several simulated used oils with different degrees of degradation were prepared by changing the molar ratio of [CHP]/[ZnDTP]. Hydro-refined mineral oil was used as a base oil without other additives. Changing the degree of degradation simulated the gradual change in deterioration during actual use. Fresh ZnDTP-containing oil showed lower wear and higher friction than the base oil. However, the anti-wear performance gradually decreased with increasing ZnDTP degradation, and an appreciable wear increase was observed after dissipation of ZnDTP. Beyond this point, further degraded oils showed larger wear and lower friction than those with the additive-free base oil. In other words, simulated used-oils were found to accelerate wear but to decrease friction. ZnDTP produced various kinds of phosphorous-containing compounds after reacting with CHP. Some degradation products from ZnDTP accelerated what appeared to be corrosive wear that might have been due to the excess reaction of surfaces with sulfur in the degraded compounds.