I did a search and was unable to find a straight answer to the problem that I am having. I have a '95 Z28 with 3.23 gears. A while ago (~10K miles), I changed the rear end fluid. I used the Valvoline 80W90 synthetic blend with the Auto X (I think) additive. I think I put too much in, but I'm not sure.
The problem that I am having is a very loud whine from the rear end. It gets loud at about 65mph and stays that way at 80 mph (on the interstate). My bro in laws '95 doesn't make any noise compared to mine. I am going to change the fluid and inspect everything today, and my question is this;
do I use standard fluid with additive, or just synthetic without additive? Everyone seems to have a different take on this.

 

You're supposed to fill it to the top, so you didn't overfill.

My advice is to change it again, using conventional oil and the Posi additive you get from GM dealers. This is how it comes from the factory.

 

When I changed mine I used Mobile 1 synthetic and the gm additive. I have had no problems with this combo. I also don't think you can over fill the rear end because the excess would flow out of the plug hole but I may be wrong. I was told to fill it until it started to flow out of the plug hole and then put the plug it in.

 

My car did nearly the same thing. I had the fluid changed with synthetic, it made a lot of noise. Changed it 7k miles later with regular and posi additive, and now it doesnt make any noise. Hope that helps

Cameron

 

Well I changed the fluid and it does sound better. I did find something interesting though. My pinion seal at the tranny tailshaft makes a squeaking noise when I turn the rear wheels. It has been leaking, so I think that I will replace that. I'm also going to get rid of the poly transmission mount as well. That thing is waaaaaay to stiff.
I also found something else that was interesting. In a past post, someone said that the ratio will be marked on the ring gear. Well, I took a look at mine and found a mark that read M3:42 on the ring gear, and one that looked like 13:42 on the pinion. Does this mean that I actually have 3:42's in the car???? If so, that'd be awesome since I wanted to put them in anyway.

 

Anyone know?

 

You do have 3.23's because the numbers on the gears were 42 and 13. 42 divided by 13 = 3.23

 

Oh, well hell that makes sense! That's for the info, I didn't know that you could figure it out that way.

 

Pinion seal failures aren't too uncommon on these rears. Had mine fixed years ago, while the car was still under warranty, 'cause it was making noise. I also know of a guy with a '97 TA that had the same prob.