GL-5 gear oils are for gears like our differentials where the pinion is below the centerline of the ring gear. You can look at the outside of the differential housing on the front side and see that. There is much sliding on the gear teeth, and the oil has to be blended for that. These are called hypoid gears. A gearbox without hypoid gears, like a transmission, does not need this particular additive package in the oil. GL-4 gear oil works well. Some GL-5 oils used a sulfur based additive to make them GL-5, and the sulfur would damage yellow metals...brass, bronze, & copper...in the transmission. Many modern GL-5 gear oils do not have the sulfur in them and are labeled for both GL-4 and GL-5 applications.