In order to clarify the effects of different levels of Type II synthetic oil seal compatibility on aero-engines, the defects of High Thermal Stability Grade (HTS) in oil seal compatibility were found through the interpretation of the MIL-PRF-23699 and SAE-AS-5780.The experiment of soaking the aero-engine rubber seal in different oils indicates that the seal in the High Thermal Stability Grade is more likely to cause expansion and degradation at temperatures in excess of 120 degrees Celsius and 1800 hours. The analysis of CFM56-5B/-7B NO.3 bearing forward seal PTFE disbond fault shows that PTFE disbond is related to the HTS oil, and it leads to engine pollution and oil leakage.