FWIW, Several OEMs went to finned aluminum covers as they went up in power and tow ratings. I use a minimum of syn 75w140 in the rear diff. RP 85w140 has done the best for me in 3 tow vehicles... the best UOAs,longest life and zero wear metal on the magnets. Try whatever you want but the most important thing is to watch for metal on the magnet. You can't even trust UOA if the metal is on the magnet and not in the oil sample... BTDT! No metal on the magnet is the most important and UOA tell me how long the additives hold up. My AAM 11.5 runs 247* and the NV5600 runs 237* towing on a slightly warm day at about 65 mph. Craig
