Generally synthetic gear oils have superior oxidation stability compared to mineral gear oils, although there is a concern that the oil life may depend on the sludge formation during its oxidation process. In the methods of ASTM D2893 or ASTM D5763 which are commonly used as oxidation tests of gear oils, the test duration for degradation is specified 312 hours. Considering the evaluation to select synthetic gear oils, the test duration is too short to find the significant difference since the degradation of synthetic gear oils would be minimal. The authors provide a new method, in which gear oils are oxidized and degraded by modified dry-TOST (dry-Turbine Oil Oxidation Stability Test) method without catalysts, and use the RPVOT (Rotating Pressure Vessel Oxidation Test) residual ratio as the newly introduced degradation index. This study shows the RPVOT residual ratio has a correlation with changes of kinematic viscosity or acid number which are generally used as the degradation index, and also presents that degradation tendency of the gear oils can be classified by the relationship graph between RPVOT residual ratio and sludge quantity.