Yes the dodge manual for dana axle recommendation was if you tow heavy to use 75-140 weight gear lube.

One question, someone said the AAM axle has a reuseable gasket, it this true?

 

Yes the gaskets are very nice. I reused the one on my front axle. Of course, the Mag-Hytec came with an "O-Ring" built into it, so I didn't need to reuse the rear one. I would have used it though had I put my stock cover back on.

They are made out of some sort of molded rubber. Nice.

 

If the owner's manual offers a heavier viscosity oil option, the manufacturer has to use that weight oil for their CAFE testing. I realize our trucks aren't EPA tested for mileage but none the less Corporate Average Fuel Efficiency may include our trucks. So they only list the lighter more fuel efficient lubes in the owner's manual. Ford got into a scwabble with the EPA over this a while back. So now the owner's manual "recommends" the light weight stuff. In my truck, 5w-20 in the motor and 75w-90 in the rear end towing over 10,000 lbs in the Florida heat causes me some concern but so does going against the warranty holder's "recommendation".