I've used Royal Purple for years and the only thing I've ever done is use a break-in oil for the first 500 miles, then drain it and refill with Royal Purple. On transmissions, transfer cases, and differentials, I always start out with this oil and have never had any problems. We recently torn down our 1986 Ford diesel 4X4 1-ton with 355,000 miles and all components were in great shape. My transmission rebuilder said that the trans., and transfer case didn't need to be rebuilt but we did it just because. We didn't rebuild the differentials because there wasn't any significant wear. The engine was also in good shape, but rebuilt it just because of the high mileage. Over the years I've had dozons of military vehicles and always use Royal Purple. On gas engines we change it at 5,000 miles and on the diesels we change at 10,000 miles.