I've always towed trailers 5500lbs or lighter. I now have a 5th Wheel that will be over 10k when loaded. We run long miles on our vacations and have gone in over 100 degree temps from time to time. Is there a table or something in writing about when to upgrade the fluid in the rearend to 75w-140. I can't find anything in the owners manuals but I know I've heard/seen it before. I've changed the fluid out to Mobil 1 75w-90 after the breakin period and just want to know if I need to change again. Thanks

i would not worry about it. how many miles you gonna two in extreme heat ? 30-50K ? the mobil 1 will be fine.