Different Lubrication
The differences in gear design create the need for significantly different lubrication designs, which is why manual transmissions sometimes use much different lubrication than differentials. For instance, hypoid gears normally seen in automotive differentials require API GL-5 concentration and performance of extreme pressure additives because of their spiral sliding action. For everyday driving API GL-5 performance and SAE 75W-90 viscosity is recommended. Heavy towing or hauling may require the use of API GL-5, 75W-140 viscosity since pressure between the ring and pinion gears are elevated.
As for manual transmission gearing, how they are set up and the service factor dictates the use of many different oils. OEMs sometimes recommend automatic transmission fluid such as MERCON or ATF+4, specialty lubes such as synchromesh fluids and API GL-4, 75w-90 viscosity gear lube. The difference in GL-4 and GL-5 is that GL-4 gear lubes have half the extreme pressure additives of GL-5. Because the gear types in manual transmissions do not necessitate the use of GL-5 gear lube, GL-4 is the correct recommendation called for by most OEM’s when gear lube is required.