ive run mobile 1, valvoline and lucas at different points and saw no change in noise from the rear end. Did you maybe use the wrong viscosity? did you add any slip modifiers or does the fluid you purchased contain any?I replaced my differential fluid with Valvoline full synthetic oil and now my differential is noisy. It drives me nuts.
Should I just put AC Delco gear oil back in?
What oil are you using?
2015.5 GMC 3500 dually.
so it depends on the blend in the bottle, my understanding, and im sure ill be corrected if im wrong, is the 15's still have the G80 locker in the rear diff, which is designed to run without the addition of friction modifiers. If the fluid you purchased has a friction modifier pre blended into it, that might be causing the clutches to slip and make noise. That said, ive always run off the shelf standard gear oil and just not added any extra friction modifier and been fine. Can you describe the noise its making, grinding, clunking ect?I used 75w90. It's for limited slip differential.