The chemical and physical properties of gear oils in use may change more or less depending on its formulation and the operating conditions. For this reason a gear oil was investigated after three years of use in a wind turbine to find out if changes are evident and if the protection of the gears and rolling bearings still meet the requirements as with fresh oil. Beside chemical and physical analyses@ the used gear oil was examined on a FZG back-to-back gear test rig and on a FE8 test rig. The test results could show that the used gear oil as well as its ability to protect the gears and rolling bearings has changed very little compared to fresh oil.