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. Analyses of the test specimens by means of Fourier Transform Infrared Spectroscopy (FT-IR) revealed that the silicone rubbers had degraded, causing changes in the polymer structure. The test specimen immersed in sample oil containing primary ZDDP was degraded to a deeper section than that immersed in secondary-ZDDP-containing oil. This may be because primary ZDDP reduces tensile strength more than secondary ZDDP. In addition, analyses of the sample oils were conducted in order to determine which of the ZDDP-derived species were degrading the silicone rubber. The phosphorus compounds in the sample oils after immersion were identified by means of 31P NMR. When silicone rubber specimens were immersed in sample oils containing these phosphorus compounds, it was found that acidic phosphorus compounds, such as alkyl acid phosphate and phosphorothioate, had major effects. It would appear that these are the compounds which degrade the silicone rubber. Silicone rubber specimens were also immersed in test oils containing a detergent together with ZDDP or an acidic phosphorus compound. In these cases, the drop in tensile strength of the silicone rubber was much smaller. The reason may be that the overbased salt in the detergent neutralizes the acidic phosphorus compound to help prevent degradation of the silicone polymer.