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80W90 IS for heavy and severe duty. Up until about 10 years ago that's what most heavy truck fleets were using. A lot of drivers I know had premature bearing failure using 75W140 syn oil in there diffs and transmissions.



Working at a dealer I would say that you mixed those two up. We saw a ton of rear gear failures in the 1500 trucks. Nothing tests a rear gear like and overzealous Californian towing his huge boat across the 120+ degree desert to get to the Colorado River. 75-140 worked way better. I know that in my 95 1500 when I would tow my enclosed car trailer in the summer I would get weapage around that lil rubber fill plug from the heat. Switched to 75/140 and it never happened again. ......as always, FWIW.