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Effect of base stock material, especially polybutene, on two-cycle engine oil performance was studied. At first, visible smoke tests were conducted, using test oils of different base stocks. Polybutene showed the best performance among the test oils. Then, piston detergency tests were performed. Test oil of higher molecular weight polybutene showed poorer results. In addition, piston deposits were analyzed and it was found that polybutene molecular weight was increased during the test. Piston detergency was discussed in relation with the increase in viscosity caused by increase in molecular weight. Finally, engine startability tests were carried out. Good correlation with viscosity was found only in test oils of same mineral base stock concentration. The increase in engine start torque was attributed to increase in polybutene viscosity caused by selective evaporation of the lower molecular weight portion.