GS977

i want to change the rear end oil before i go back to the track friday. i don't think it's ever been changed since it left bowling green in 82. question is do i need the GM posi additive if i use synthetic gear oil. and what's the thinnest gear lube weight i can use. i'm looking for the lowest drag.

Ralphbf

Just went through this.

Regular oil, non synthetic is recommended, 75-90.

The idea it to allow your limited slip work around corners.
Synthetic allows it to slip in a straight line.

Some even put in a partial bottle of GM's limited slip additive and if it still makes noise in a corner they add more.

Royal Purple does not require any additive nor does Mobil 1 LS.
But are they too slippery is the question.

I stayed with regular oil and GM's additive.


Ralph

GS977

thanks for the info ralph!! i'll pick up a bottle of add from chevrolet tomorrow. and non synthetic.

7T1vette

Hooey on the comments about not using synthetic gear oil in the rear end. GM didn't recommend it because it wasn't readily available in 1982. Later on, GM didn't 'revise' oil specs for the C3's because they were over 10 years old.

There is NO documentation that synthetic oil would be problematic for the rear gears; just like there is NO documentation that it would be OK for [C3] rear gears. Basically, there is NO documentation either way.

But, other CF members have used synthetic oil in the differential for years without any problem. So, the choice is up to the owner.

There is no question that gears and wearing parts live a much longer life when lubricated with [good] synthetic oils/grease, instead of the organic stuff. Either will work fine, IMO. But, you still need the Posi-trac additive if you choose synthetic.

RonR80

I still use the old 80-90 in mine but definitly add the "additive" I even add a little GEAR-UP in it

capevettes

I've been using 80/90 Gear weight oil and 1 4 oz bottle of GM posi lube in the rear ends of my 61, 65 and 68 for many years (41 years in my 65). They are all original rear ends that have never been out of the cars. It works

iokepakai

Using Royal Purple,no problems since change 8 months ago.Has the friction modifier added in already......

GS977

good info guys!!!i saw the royal purple and almost bought it. but i had already bought the GM additive. so i went with regular 80-90 weight i think. i could only get @ a pint, maybe less out of it. it needed a oil change bad. i only bought 2 quarts and it took almost all of it.

Redwing01

100%

Ralphbf

I love synthetic oils. Ran Mobil 1 since 1974 or so.
But in the rear end the old stuff seems to be just fine.

Like capevettes said 41 years and still going strong.

And there is still the valid question about synthetic being too slippery for these rear ends.

May not of made the perfect choice but it's still a good one.

Ralph

capevettes

Don't forget to do some figure eights to work it into the clutches

hugie82

I used synthetic in an old posi and every seal started leaking with in a month. After rear is rebuilt than use it.

jim-81

a little off topic but...

Would going to a Quick Lube place be able to do this (change the diff oil with the proper sfuff)? I take my regular car there and they are pretty thorough. I also don't have a 'sucker' to get the old lube out and would like to change it out.

Thanks, Jim

Ralphbf

They will have something they say will work, I picked mine up at GM and had them do the change.

Ralph

hugie82

I had an STS tire store do mine once but you have to watch them and give them the additive or you'll be waiting for their parts guy to pick it up.