This is an article on building up a HD towing rear axle in a Ford truck that I bookmarked several years ago when I had a '95 F350. I've always thought it had some pretty good info in it that they've backed up by real world testing.
Note the gauge verified temperature reduction with the Mag Hytech diff cover. Also note which non-OEM oil produced the largest reduction in differential temperature (not Amzoil, although Amzoil lowered the temp a respectable amount as well).
FourWheeler HD Towing Axle Article:):)
