I haven't followed OEM recommendations on motor / gear oil brands for over 30 years. Our Vipers have nothing but Amsoil front to back. I put the gear oil in the tranny at 16,000 miles (5/02) and changed it once so far at 55,000 (4/06). Oil analysis showed it had a lot left to go. The rear end gear lube was put in at 32,000 miles with the new Unitrax 3.45s (4/04) and changed once as well at 55,000 miles (4/06). Oil analysis showed the same for the rear end lube...suitable for continued use. I actually think the car runs cooler with these products in there....but can't prove it. There doesn't seem to be the massive heat transfer up the MGW shift knob there used to be when idling in parades or heavy traffic.
Bottom line is that if you trust the product, and it meets the outlined OEM specs, there's no reason not to use whatever you like. As far as I know, no warranty can be voided by using any product that meets or exceeds OEM specs. I think manufacturer drain intervals need to be followed while under warranty.
Steve