A 2.5 MW wind turbine gearbox design was considered to perform a power loss prediction using different wind turbine gear oil formulations. A gearbox power loss model, previously validated with experimental results, was used to predict the efficiency of a full wind turbine planetary gearbox. The power loss model account the gears and rolling bearing losses using well established models calibrated with a method proposed by the author. The calculations clearly showed that significant energy savings can be achieved by selecting different base oils, modifying gear tooth geometry, or combining both.