A bench test was performed with Toyota 1500 cc OHC engines using a modified AMA mode in order to determine the change in performance, such as oxidation stability, thermal stability and antiwear performance, of gasoline engine oils as a consequence of their degradation. For the purpose of analyzing the relationship between the degradation of engine oils and their performance in detail, engine tests were performed without any new oil supply. The remaining performance of used oils was discussed in connection with both the degradation of engine oils and the depletion of oxidation inhibitors.The oxidation inhibitors, such as ZDTP, diarylamine and high molecular weight phenol, remaining in used oils were measured quantitatively with liquid chromatography. Oxidation stability, thermal stability and antiwear performance were measured with TFOUT, the hot tube test and the four ball test, respectively.It was found that the higher the temperature, the larger the rate of depletion of the oxidation inhibitors. In particular, the duration at high revolution speeds in which the sump temperature was raised up to 115° C accelerated the depletion of the oxidation inhibitors, resulting in the degradation of oils. The order of the rate of additive depletion was ZDTP > diarylamine > high molecular weight phenol, which corresponded to the thermal stability of these additives.The remaining oxidation stability of the used oils decreased gradually with the depletion of oxidation inhibitors; however, an adequate stability was observed even after the complete depletion of the additives. This might be explained as that the decomposition products of ZDTP could also act as an oxidation inhibitor in oils. The antiwear performance decreased rapidly within 50 h of the test and then decreased with the increase in the total acid number of the oils, probably because of corrosive wear. The thermal stability also decreased slightly around 150 h and then decreased rapidly when the total base number of the used oil dropped to less than 2.5 mgKOH/g.