neighbors truck, 01 F350 2wd dually with the 7.3, 241k miles. He's never been one for meticulous maintenance, regular oil and fuel filter changes, tranny fluid change every 100k, but not much else. It hasnt given him a lot of trouble the past 17 years. Lately hes been recovering cypress logs out of the swamp, having them milled, and towing them to buyers. Due to all the weight hes been towing, I asked if he ever changed the oil in the rear end. He said no, I checked the level for him it was low, and had a ton of metal on the fill plug magnet. I added some oil, and we plan to clean it out and change it soon. From reading online and on Amsoils website, it shows the Dana 80 to take 75W90. I found that odd being the Sterling 10.5 in my F250 and most other ford trucks require 75W140. Should we stick with the 75W90 or step it up to 75W110 or 75W140? When the trailer is loaded down with logs it probably weighs around 9-10,000 pounds, and he towsno slower than 75-80 mph, no trailer he normally runs 80-85 when he can, the guy is always in a hurry, so I figured the rear end sees a good bit of heat. The truck doesnt live an easy life but it just keeps on ticking. Thanks for any suggestions.