Christophe2m6

Hi guys!

Will replace the fluid in my 1969 Cutlass Supreme 12 Bolts differential.
The car has the 350ci 4bbl with auto trans.

I've read some posts on the forum but as I live in France, would just need the confirmation that I can use this fluid:

LUCAS MINERAL 80W90 OIL SAE


Thanks a lot for your help!!
Chris

toymobile

Unless you have a POSI that will work just fine, POSI will need an additive along with that lube.

Johnny

Christophe2m6

Thanks for your answer Johnny!!

Unfortunately, I don't know if I have a POSI or not...

The only things I know is that I don't have a "tab" attached to one of the 12 bolts, and sometimes when I hit the gas pedal hard, just only one wheel will "burnout"!

Does that help!?

Fun71

Your description sound like an open differential. If you actually have a limited slip diff, the above fluid without the additive will work but there may be some chattering/clunking during turns. If that happens, just put the additive in.

Christophe2m6

Thanks a lot Fun!
I ordered 3 qts of the above lube.
If I actually had to use the additive, which quantity would I need?
Thanks a lot for all your precious help!

Fun71

It has been MANY years since I bought the additive, but I seem to remember a small bottle (6 or 8 ounces) was all it took.

redoldsman

Interesting. I never knew you could use and additive for a posi. I always thought you had to use special fluid for a posi. Just proves we never quit learning.

Fun71

Well, GM sells (or used to sell) a specific fluid for Limited Slip Differentials. I bought some from the dealer back in the 80s the first time I worked on the rear on my Cutlass, but I still had to buy a bottle of additive. Back then I didn't know you could use regular gear oil with the additive.

toymobile

The additive smells like SKUNK OIL, or at least just as BAD.

I found that only GM sold an additive that worked, Tried one from FORD and had to change it out, still had chatter, bought GM and it worked, Should have known better but LIVE AND LEARN.

Johnny

RROLDSX

At one time the additive was made from whale oil. Can't recall what it's made from now (smells because of some type of fat).
It allows the plates to slip a little so when you found a corner you don't get the chatter
caused by the two wheels trying to go different speeds.