Well, a mixture with GL-5 and GL-4 can be used in the gearbox. You can also use a mixture of MT-1 and GL-5 to put into the gearbox. But all the engineers recommend not to use the GL-5 in the engine. Even if you the mixture with GL-4, there will always be a possibility that it might damage the bronze parts in a gearbox.

Can I use GL-5 instead of GL4?

No, the API GL5 rating does not supercede or cover the GL4 rating. It is important to follow the gearbox or diff manufacturers recommendations and use the correct GL rating.

Can I use GL5 in a GL4 transmission?

It is a common belief that API GL-5 oils can be used where API GL-4 requirements are specified. Although this is true for “gear oil”, it does not make API GL-5 gear oils satisfactory for all transmissions especially those that use synchromesh.