I used 75w90. It's for limited slip differential.
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?
2015 Chevy Silverado 2500 HD 6.6L LML