Love to read threads like this one. I'm always learning and when the time comes I can always refer back to it. Thanks for posting.

Be careful with the synthetic fluids. Not all of them hav ehte additive included. I know that Mobile 1 doesn't.

The Mobil 1 LS 75W-90 gear oil does have the additive already. I looked at that one too.

http://www.mobiloil.com/USA-English/..._75W-90LS.aspx

Mobil 1 Synthetic Gear Lube LS 75W-90 does not have enough LS additive to meet GM spec #88862624. This is why I went back wit the GM fluid. I didn't want to use more additive, nor add more than necessary. I think Valvolines synthetic gear lube meets this spec.

To the o.p. The owner's manual specifically says to change your diff fluid after every track day. If you tracked it a while ago...and just now got around to changing your fluid...that's what could have caused the chatter.

I had the chatter in my SS, but I got my techs at work to upgrade me to the LS gear oil with new seals and all is well!:thumbsup:

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So how is everyone's diff doing now?
Does the valvoline fluid work fine over the lung run wihout needing to use the LS additive?

The original poster added some extra addative to the valvoline oil eventually.

here is my .02 on this topic take it for what it is worth. buy some good fluid. IE the gm fluid, good aftermaket synthetic. Now if the fluid has addative don't run any extra up front. If you buy fluid without the additive use the recomnended GM addtive right out the gate.

after you fill the diff do some figure eight driving in a parking lot to cycle the diff clutches. I think the GM TSB specifies how many to do

Now if you still get some chatter go get more of the GM addative that is recomended at add say 2 ounces at a time then go repeat the figure 8's until the chatter is elimitnated.

the friction modifer material is not a more is better type thing you need the right amount.

my fresh gm fluid will be here this week I will do the change and if I get a bit of chatter I will do just what I posted. I am positive it will work because it worked on my Z06.

my chatter is very minimal on my car right now and only when cold but I want it 100% gone. sometimes I think with these tires having such piss poor grip in the cold it magnifies the issue.

I tried the Mobile 1 in my car. Definitely not enough additive in it. When the car is cold, the differential makes more noise than when it had the GM fluid. I'm gonna try Valvoline next.

Unfortunately, Mobil One doesn't meet the spec. The GM/DELCO fluid was expensive but worked well.

There is now a bulletin out on this, it calls for the dealer to flush and fill three times along with some figure 8s, etc. I had mine done and it silenced the howling.

I think my ZL1 has this problem. I just didn't know what the hell it was.

Don't know when the bulletin came out or whether my dealer did all that, but they pulled it apart to check it last year and refilled it and mine has been fine since. Basically what this guy said.

I just came back from my dealer and cant believe they were going to put the wrong fluid in the car. The tech looked at me like "who the hell is this guy" I told him that this car uses a fluid from gm with the additive already in the car. He went to parts and came back with the right stuff.

When filling he used 2 bottles/quarts of the fluid and pumped it in then I reminded him that in needs 2.1 and he again went and got another bottle and pumped in about 1/2. Some fluid came out and we watched it drain from the fill plug and closed it up when it started to drip.

Makes me thank this forum and its memebers for the insight and info. I pray this guy torqed the bolts right.

Now that I think about it i hope he cant overfill the thing........