Well, it's about that time. Have to change the diff oil.

I have the oil, but someone told me about a special limited slip additive that GM sells? Anyone familiar with it? Something I need to get before I get the car up on stands?
--Yak

 

Yes it's recommended. Other lubricants such as Royal Purple, Amsoil and Redline come with the additive in it. I'm changing next week using the Royal Purple also in my A4 transmission with the filter change.

 

It is a positraction fluid that modifies the friction.
I would put it in (1 - 4 oz bottle) regardless of whose oil you use.
Yes I know that some manufacturers claim you don't need it, but I have also read posts by forum members who did not use it with those oils and ended up with rear diff noises. When they subsequently jacke dthe car and added the 4 ounces... the noise went away..

If you need to do it and the GM dealer is closed, you can buy after market posi- traction additive at most autoparts store...

 

Yak,
Here is AMSOIL's discussion on why limited slip differentials need the friction modifier:
"Limited-slip differentials allow different wheel speeds when turning, but when one wheel loses traction, greater torque is transferred to the wheel offering more resistance. Limited-slip differentials make use of friction between clutch plates, as opposed to the meshing teeth employed by locking differentials. Transferring less torque per side, limited-slip differentials are generally smoother and quieter than locking differentials, but still sometimes "chatter" when transferring torque to the wheel with traction. In some differential applications, a friction modifier additive is necessary to reduce chatter."

They add this note:
NOTE: AMSOIL Synthetic Gear Lubes do not require the use of this additive. However, not all differentials respond the same and if chatter is noticed, the addition of AMSOIL Slip-Lock Differential Additive will eliminate it.

So, to really answer your question as to whether you need a bottle, we would need to know what fluid you are using, because as mentioned, many top fluids come with the friction modifier in them. FYI, too much friction modifier isn't necessarily a good thing either. With too much friction modifier, the diff friction plates won't grab as designed resulting in a diff with less limited slip than designed. This will send more torque to the tire without traction resulting in slower acceleration off corners and hurt acceleration which results in wheelspin.

 

Thanks for the info.

I'm using the Amsoil fluid; Severe Gear I believe it's called.

 

Yak,
Yep, the AMSOIL Severe Gear Extreme Pressure Synthetic 75w90 is the same fluid I use. As this fluid does come with the friction modifier in it, I don't add any and my diff doesn't chatter. If you do find your diff chatters, you can add a bottle of friction modifier, either GMs or the AMSOIL version I linked above.

 

I'm running Royal Purple in the diff with no friction modifier. No noises or problems.

 

If it makes you feel better, you can add it with no ill effect. Pick it up at any GM dealer. Many do not need it(RP, Amsoil, Redline etc.)

 

After you change the fluid you must drive the car in a series of figure 8's to work the fluid in the rear diff. this will eliminate any dry spots after the fluid is changed.

 

Probably worth mentioning that the service manual states something along the lines of not using WOT for 300 miles after fluid change. I can't meet that with mine, so I do a couple tight turns in the paddock and hit the track.

 

Thanks for that information. I'm changing mine this week to Royal Purple.
I just came from changing the tranny fluid and filter (A4) using Royal Purple in it also.
Rsty

 

Just serviced my tranny A4. Changed filter and fluid to RP. Noticeable differance already...diff is next

 

How many miles should you go between trans and diff changes?

 

50K on A4 before fluid change..change filter also if driven serverly.
Diff I'm not sure ..but changing it now also.
Rsty

 

Thanks!

 

Amsoil in all of my rides!

 

Me too!

 

having mine changed to RP tomorrow. Car will have all RP products..engine, tranny and diff.
Wife's Jeep already setup that way and so far so good. She is geetting 1-3 mpg more