Before I start, Id like to let everyone know that I did search.. and for a quite a while. I read through numerous threads however most were very old so I am hoping some people can give some updated info, especially because there seems to be a lot of new Z06 owners on the board heres like myself.

I am looking to change the rear differential fluid in my car. It has around 40,000 miles on it and has never been changed. The car developed a slight whine which is pretty inconsistent and Id like to at least change all the fluids so I know it has the right stuff (and new stuff) in it. It looks like the common choices are all synthetic 75W-90 from these vendors:

Amsoil Severe Gear Fluid
Royal Purple
Mobil 1
Redline

It seemed to be a large argument whether the limited slip additive available through GM was needed to be added to these fluids since most (or all) say they contain a limited slip additive.

I just want to know some input on this from people who really know their stuff and why some are better than the others, not just the, "I use product A and it works great." Thanks everyone!

 

Well, I am not an expert like some around here; however I will step up and say that the Amsoil Severe Gear fluid does NOT require adding the limited slip additive. Can't speak about the others.

 

Welcome aboard, and it's a breath of fresh air that you have done some research on your own. I've used this forum from day one to learn different things and preferences of current vette owners. I'm just getting ready to back my car off the ramps for the first time since last Oct. I just changed out the clutch fluid(Prestone Dot4) diff oil( Redline 75/90 High Performance all synthetic gear oil, 2 US qts., I bought the diff from LG Motorsports , of Texas, over the winter. They recommended using this oil + 4 oz bottle of Red Line Limited Slip differential modifier. break in additive. I mixed the additive to the 2 qts of the gear oil in a gallon jug and that gives you a little more than what it will take. I also changed out the trans fluid(M12) w/ 1 gallon of Red line D4 ATF Auto/Manual transmission/trans axle fluid. All Synthetic. Changed engine oil w/ Mobil 1 5w30 syn (6 1/2 qts) along w/ a Mobil 1 #107 oil filter. I also install a ECS trans/ diff strut brace. I bought all my fluids from Jegs, over the phone, and they delivered within 48 hrs. I didn,t feel like running around the area to find these products. Well, I'm getting ready to take a little ride to see if the 3: 90 gears feel any different than the 3:42's I took out. Hope this helps, and have a great day.

 

The RP label says it contains a limited slip additive.I use it in my 02Z and 03S2K.

\db2
Carolina

 

Honestly, I've never used anything but RP ever since I got my car. During oil changes, it does seem to run better. Here, it's more based on preference. RP, Amsoil, Mobil, Redline... you can't go wrong with either of them, I'd say.

 

I replaced all my major fluids in my 02 Z over the past 2 months with RL products. The 75-90 Diff oil doesn't not contain any slip additives and I've not had any lockup problems with it out on the autocross course, nor on the street.

 

ive used the amsoil sevre gear and both times did NOT have to use additive.

 

I use the AMSOIL Severe Gear Extreme Pressure Synthetic 75w90 in both my 02 Z06 track car and my 06 CTS-V daily driver without extra additive. This fluid comes with some additive already in it and the older GM diffs don't seem to need extra additive to prevent diff chatter. That said, the C6 diff does and for the C6 I recommend adding a bottle of AMSOIL Slip-Lock Differential Additive.

For the DIYer, this comes in handy:
AMSOIL Hand Pump

 

As I understand you use AMSOIL for severe duty conditions. I had a problem when GM changed my oil to "regular" oil without my permission but they did it anyhow. Afterwards I got problems changing from 4th gear to fifth on track when rpm went up to 8900. I figured out that shifting problem was due to the oil. Now I want to close this lottery and get back to the right oil and not use GM services anymore.
What oil do you use? Your recommendation comes without liabilities so what I am asking is just the type and brand name. The rest is up to me.
I just need right type of your experience of oil you use to avoid more damages to the gears that I have.
With GM oil I got damages when trying to change from 4th gear to 5th gear under 8900 rpm. Originally I think I had RP oil by the tuner (Cartek in NJ). I apprciate your feedback.
Thanks.

 

AMSOIL makes two transmission fluids that they recommend for Dexron III applications. I use both.

AMSOIL ATF (product code ATFQT)
AMSOIL Torque-Drive Synthetic Transmission Fluid (product code ATD1G)

A little explanation. The ATF originally had a hot viscosity of about 7.3 cSt and I have been using this fluid with excellent results in my 2002 Z06 on the track for years. No failures.

About a year and a half ago, AMSOIL dropped the viscosity of the ATF down to 6.8 cSt hot so as to make it suitable for both Dexron III (GM manuals) and Dexron VI (GM autos). Given this, AMSOIL swapped their primary recommendation for GM manuals from the ATF to the ATD. I had the ATF in my daily driver 06 CTS-V (same Tremex 6 speed tranny as the Vette), but earlier this year I swapped my Caddy to the ATD so I could try it as I would be recommending it to customers. After the swap, I really couldn't tell the difference. The ATD has a hot viscosity of 7.4 cSt and no viscosity improvers so I feel that this is the better choice for our six speed manual trannies.

While I was at sea, AMSOIL increased the viscosity of the ATF up to 7.6 cSt and no longer recommends this fluid for Dexron VI applications, it is stricly a Dexron III fluid again, much like the ATF I have been running in my Z06 for years.

However, since the ATD has no viscosity modifiers and thus is totally shear stable, I feel that this is the better choice for the track and for your high HP application.

Hope that helps!