I just did 30K Rear LSD Diff and went with Mobil 1 SYN 75W90 and did not add any modifiers since i called Mobil and they said it did not need anymore? BIG BS on that my rear end was growling like a power sterring pump with no fluid in it! Sound was coming from rt front of truck it seemed but dealer drained and put dino valvoline 80W90 with modifiers in rear diff instead of draining out enough fluid to just add modifier to exisiting fluid, noise has gone away and it was no chattering in this noise it was a loud growling and glad it's gone!
