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In developing engine oils, it is crucial to consider their compatibility with the rubbers used for seals. Among the different seal rubbers, silicone rubber is particularly susceptible to attack by acids and bases, which means it would be more likely to be affected by certain engine oil additives. In this study, the effects of some major additives, namely detergents, zinc dialkyl-dithiophopshate (ZDDP) and molybdenum dithiocarbamate (MoDTC), on silicone rubber were investigated. Silicone rubber test specimens were immersed in sample oils containing these additives for a prescribed period at 150°C, then the physical properties of the test specimens were measured to compare the effects of the different additives. It was found that ZDDPs dramatically reduce the tensile strength of silicone rubber, with primary ZDDP having a greater effect than secondary ZDDP.