Bond007

Hello Fellow Vetteheads,
While replacing tank etc checked the oil in the differential and noticed it to be very black...how does one get it out ..should it be warm?..what oil and additive do I use to replace?
Thank You in Advance.

63mako

You suck it out with a suction pump. Kind of a hassle to get it all out.

Redwing01

I just picked up a fluid transfer pump to do mine.
Princess auto 14 bucks
You should use genuine GM differential fluid and make sure to pick up a small posi additive as well.
Take the car for a short drive to warm up the fluid first, makes it easier to get it all out .
When I did mine I put the new fluid in a pail of hot water before opening them up to make putting in the new lube much easier
Good luck.

Vette5.5

I'd be careful with the friction modifier additive. It's function is to allow more posi clutch plate slippage around tight turns, to help quiet things down. Problem is, stuff sometimes takes away a good amount straight line posi clutch pack lock up action, when you're really wanting it. Never use it myself. You can always add it later.

pws69

If that is the case, then your POSI has issues!

Use of the POSI additive is strongly recommended. 1 bottle per change.

Redwing01

Even do yourself a favor and do a "search" on this forum regarding differential fluid changes and you will see for yourself that the addition of a 4oz bottle of posi fluid is reccommended in virtually every post.

63mako

For the last 10 years I have used Amsoil 80W-90. It has the friction modifiers in it. Absolutely zero issues with no additive and the posi works fine.

Jud Chapin

There's another way to drain the differential which makes the job much easier. Install a drain plug. Have a look...

jb78L-82

75W-90 Mobil 1 synthetic with no friction modifier for 22 years now with zero issues. From my experience and others who have chimed in here and many times when this issue has come up in the past with fluid for the differential, it strongly appears that the friction modifier is NOT needed with synthetic gear oil. All of the fluids for these cars are so much better than 30-40 years ago with the advent of synthetic fluids that following the manufacturer recommendations from that era seems a little archaic, IMHO. Whatever you choose to do will work as long as there is nothing wrong with the diff.

C66 Racing

Here is what 63mako uses:
AMSOIL Synthetic Gear Lube SAE 80w90 (Product Code AGLQT)

More than happy to the above for forum members at dealer wholesale pricing, about 25% below retail, via the AMSOIL Preferred Customer Program. Drop me a PM if interested.

Bond007

WOW MORE CONFUSED THAN EVER
WILL PLAY IT SAFE AND GO WITH THE ADDITIVE.
THANKS EVERYONE.

Vette5.5

As mentioned, you can always add stuff later, but not so easy taking it back out. My C6 ZO6 has a pretty noisy rear posi diff around tight turns, getting many owners to add the GM friction modifier. Really doesn't bother me, as road track the car, wanting all the perfomance advantage possible. Car came from the factory this way, and really not one to second guess the GM engineers.

Street Rat

I just changed rear end fluid today. You will need a device to suck the old fluid out. Also a device to put the new fluid in. I used a small pump that screws on to the gear oil bottles. I attached a clear hose to the pump so that I could see the fluid going in. Pump the fluid in until it starts to dribble out of the plug opening. I dropped my spare tire to give me better access to the fill plug. Everything should be easily available at your local auto parts store. By the way I used Amsoil synthetic Severe Duty Gear oil AND the Amsoil Limited Slip additive.

jordan89

75W-90 severe gear AMSOIL here. At least that's what will be going in this week. I already drained the Diffy and cleaned it.

Stephen Irons

I've done this job a few times, using different options re oil.

I now have rebuilt the diff and have gone back to teh tried and tested, and in my opion best option - GM oil and GM additive. I've found it works best, and lasts longer between changes.

For you guys in the US, obtaining it is easy. More difficult for us here in Europe, but worth the hassle as far as I'm concerned!

gcusmano74

Another place to put the drain plug is under the bracket that holds the strut rods. Drill and tap 1/4" NPT just far enough to one side that you miss the ring gear. Make the hole on the bracket larger, of course, so you can easily insert/remove the plug..

Street Rat

I read somewhere that the models with aluminum housings were better left without a drilled and tapped drain plug. '80-'82s I believe. The housings tend to strip out when tightening a drain plug. Just some additional info to digest.

resdoggie

If I read correctly, this lube oil can also be used in an M-21?

jim-81

this is a good idea for a project to tackle next weekend...

Bond007

What are the correct names for the GM oil products for:
Manual Transmission
Differential
Additive

Thanks