I am planning on replacing the fluid in my 70 SS rear differential with 3:31 posi. The gasket needs to be replaced and I figure I will change the fluid while at it obviously. What fluid do I run and since it has posi, do I need to put in an additive as well? Is there a plug on the backside of the differential that I need to access to add the fluid as well? Thanks in advance. :hurray::beers:

75w-90 gear lube...likely GL 5 and the GM posi oil that you can buy at your friendly Chev dealer. BTW, this is what I run in my C5 Corvette, except I step up to the full synthetic stuff with 1.5 cans of the posi oil.

And, do change it once in awhile.

I just bought a new series 3 eaton posi from IWE rear ends only
( www.iwerearendsonly.com )and they gave me a bottle of the Motorcraft posi additive. They said it is way better than the GM stuff

Some Synthetic rear end fluids will not need additive, I run Amsoil and the new Posi is doing well with the straight sythetic gear oil.

I have about 500 miles on it so far and not the first fell of chatter. have run many other Posi units in the past with the same results.

If it chatters with synthetic lube, then add the Posi Additive.

-Bobby-

I have heard a number of times to do 10 or so, tight figure 8's at a low speed in an open parking lot to work the new gear oil and additive into the posi components. Wives tale or not - I did it.

I am planning on replacing the fluid in my 70 SS rear differential with 3:31 posi.

Is there a plug on the backside of the differential that I need to access to add the fluid as well? :hurray::beers:

The square ended plug on the side of the differential housing is the one that you remove to fill the case.

If it hasn't been removed in 20 or so years, expect it to be a bugger to get out. Spray it with penetrating oil and TAKE YOUR TIME.

Based on the assembly manual (which records the fill amount in lbs) the amount needed is basically 1.9 quarts. You fill it to the bottom of the fluid hole. Do not overfill.

If you are getting fluid w/o the posi additive, when you have added about half of the fluid, stop and add the posi additive so when you add the second quart you're not full before the posi additive is in. :D

You might also want to buy a short piece of 3/8 ID hose to slip over the "cut & pour" end of the bottle. It'll give you more flexibility when pouring. I bought a foot of the clear stuff at Home Depot so I could see it.

I run Royal Purple synthetic gear oil and it says on the bottle there is no need for additive...my NOS gm posi(was going to purchase a new eaton but came across the NOS gm for a steal) unit it doing well too

put 2 quarts of lakewood BFL in mine (12 bolt posi) last time. don't need the additive with this stuff..bought at the summit store for about $12 bucks a quart though..

redline oils makes a nice oil that i use, its equal to a 75- 90w but pours like water. It works good as the last set of gears i broke i let the rear drain upside down for 3 months and all the oil was still stuck to everything!It is pricey at 40 bucks a gallon though

i recomend not using synthetic oils in the eaton clutch type posi's. the oil is excellant at being too slippery and then the cluch's will not grab properly.

always tell guy's when buying my rears to use regular old dino oil 85/90w and 1 4 oz jug of gm additive. tom

i recomend not using synthetic oils in the eaton clutch type posi's. the oil is excellant at being too slippery and then the cluch's will not grab properly.

always tell guy's when buying my rears to use regular old dino oil 85/90w and 1 4 oz jug of gm additive. tom

I agree 100%.

Now the spool I ran in the race car I used Amzoil in the rear.

they gave me a bottle of the Motorcraft posi additive. They said it is way better than the GM stuff

I run Torco RGO 85w-140 in my Moser 12 bolt with one bottle of the ford additive for the same reason. I can say it stinks worse than anything I have every smelled. Makes gear oil smell pleasant!