One embodiment may include a gear assembly comprising at least one pair of meshing gears situated inside a gear case having a sump portion and a wall portion. A gear oil mixture may be disposed at least in the sump portion. At least one gear is immersed partially or fully in the oil mixture. The oil mixture comprises a hydrocarbon oil base and a coolant having an endothermic liquid/vapor transition at a temperature between the lower and upper operating temperatures of the gear assembly.