The author carried out the engine tests and laboratory experiments of engine lubricating oil, and by the latter attempted to evaluate the engine perform-ance of lubricating oils with respect to the cleanliness of pistons and others as well as the corrosion of metal parts.The dirt in engines is caused by both the lacquer for-mation due to oxidation of lubricating oils and the deposition of the soot which originates from the incomplete combustion of fuels.The lacquer forming tendency of lubricating oils is correlated with the amount of the polycyclic aromatic fraction which.is seperated from the oil by silica-gel chromatography.The deposition of the blow-by soot is influenced by the detergency of lubricating oils.For the assessment of the dete-rgency, a “ Cellite-chromato ” test and a “ carbon sedimentation test ” are available.The effects of additives and differences in refining process on the corrosion of cupper-lead bearing were investigated according to the Sohio oxidation and corrosion test procedu-re.Moreover, the cause of the corrosion of aluminium alloy piston in an engine run on.leaded gasoline was researched, and the corrosion was attributed largely to the chemi-cal action of halogenated lead.This fact coincides with the general view that in engine tests the quality of fuels affects the performance of lubricating oils deeply.By oxygen absorption tests, it was ascertained that the contribution of the above-mentioned polyc-yclic aromatic fraction to the oxidation character of lubricating oils was nearly equal to that of synthetic oxidation inhibitors.Then, the author claimed that the polycyclic aromatic fraction was the so-called“natural inhibitor”.