Lubricants used for gearboxs (manual or automatic) and the rear differential in your vehicle are different from those used for the engine. Transmission Lubricants have a life much more important. They must last 120000 miles for a truck or even the lifetime of the vehicle for some models such as Mercedes-Benz. Transmission lubricant must be chosen in accordance with the recommendation of the manufacturer of your vehicle.
A Motul 100% synthetic lubricant is a lubricant composed of synthetic base oil.