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80/90 for reg duty, 75/140 synthetic for towing and severe duty, This is from my owners manual for my 03 Dakota 9.25 limited slip 3.55. Had my rear end rebuilt at 30k under warranty started to howl bad, Then at 90k it started to growl and howl loudly again, this time i got the rebuild kit from Randys and replaced the carrier, pinion, bearings and races, the pinion seal, and crush sleeve, and axle bearings and seals. BTW i think the axle problems were do to the small vent barbed fitting being clogged i took a drill bit that was just a hair bigger than the original size and drilled it out both times the gear oil was milky because of the pluged vent




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.