1987 325E LSD Whistle

[Pro1914]

Hey folks! Just decided to do some preventative maintenance last week by changing the differential fluid, but it went very south very quick haha. Everything went smooth at first, let the oil drain all out, then filled it back up with 75W90 RedLine. The first day, I heard no noises whatsoever. Then the second day, when backing out of my parking spot at a low-speed turn, I heard a clunking/grinding noise. Not sure what it was, I just figured it was a random noise and went about my day. On the third day, anytime I went over 35MPH, the diff would sound like it was screaming for its life! Even with the windows up and the radio on, I could hear clear as day something was making a very high-pitched whine from the rear. So I took it to a parking lot and made a figure 8 at low speeds. The whole time it sounded like the titanic was sinking... Maybe it just had to break in? So I went one more day before I knew something was wrong. Jacked the car up, and checked the level, everything looked good. So I figured I'd put the old stuff back in since it wasn't making noise before. If the noise went away, it was the differential, if not, then it was something else, or I had broken the diff. But sure enough, old stuff back in, and it was dead silent for a couple of days, but now the whistle is slowly coming back. The new stuff I had just put in was pretty dark and did not look healthy at all considering I had put less than 50 miles on it. Does anyone have any idea what I did wrong? Should I run a thicker oil like 80W90, or 75W140?


TLDR; Changed differential fluid, started grinding and whistling, went back to the old dirty stuff, it doesn't grind, but sometimes whistles over 35MPH and /only/ over 35MPH. What should I do?

Thanks so much!

[hakentt]

75w90 redline is proper oil for limited slip differentials. To me sounds like something failed inside the differential, not do to oil change, but from abuse or age.