I have a 1998 GMC C3500 Chassis Cab 350 Vortec 5spd 2wd. It calls for 80w90. I do moderate to heavy towing and hauling, occasionally pulling trailers weighing about 12,000lb I was wanting to upgrade to synthetic oil and a finned aluminum diff cover. I am undecided on either 75w140 or 85w140. which one is better? I want equal or better fuel mileage, and Good rear axle protection.

Thanks for all the advice!

The advantage of synthetic over dino oil will offset the additional heat generated by heavy towing somewhat, but to go with the 140 will definately create some drag especially in colder weather before it's warmed up. I'd suggest you try 75w90 Mobil1. If it's a "posi" I'm not sure what to say about the additive (fish oil) needed for that because it would have to be compatible with the synthetic.
Hope that helps...

75-90 or 75-140 would be fine. i would not recommend going to 85-140.

Another option if not wanting to go to 140, is to use the new 110. It meets the old 90 specs, but was created in 06. They just split the viscosity range of the old 90(13.5 to 24 cSt) The new 90 is 13.5 to 18.5 and the 110 is 18.5 to 24. So either will meet the old 90 specs but the 110 will be on the higher viscosity end, giving more protection without the added drag. Hope that makes sense:)

75W140 in my last three trucks and knock on wood I have yet to have any type of differential problems.