clatgeo

How much GM limited slip additive would you add a 1970 posi rear end that holds 3.7 pints of 80w-90 gear oil

mikem350

The correct answer is one bottle of additive. Drive a little while, do some turns to exercise the clutches. If it still chatters, add one more.

fishslayer143

I would add 4-6 oz even if your Gear oil says it has it in it.

bashcraft

Sounds like you enjoy wasting money.

BEVO76

I am running Royal Purple 75w-90 with no additive.......best decision ever. That stuff is smooth as silk.

The people from Duntov refer to it as horsepower in a bottle.

fishslayer143

No, just Not wasting my time thinking the new oil will have enough for old clutches...and I won t have to put it back on the lift and I won t need to drain some oil out to make room for more friction modifier when the chatter doesn t stop . But you enjoy ...

jb78L-82

You don't need friction Modifier with the newer synthetic gear oils like Mobil 1, Valvoline, Royal Purple etc..NOT needed. Mobil 1 75W-90 now for 25 years with no friction modifier and the diff behaves perfectly....The only service I have performed is to change the oil every 7-8 years...

BEVO76

7T1vette

If you use non-synthetic gear oil, one bottle of additive is needed. But, I wouldn't put new additive in "old" oil. Suck out the old stuff and add fresh oil and additive. (You need 1-1/2 bottles of gear oil. Empty diffy; put in one bottle oil; put in the additive; fill to plug hole with some of the second bottle; install plug.)

But, I also suggest that you put synthetic gear oil in the diffy WITHOUT the additive. MUCH better lubricitiy and reduced frictional losses.

ChevyObsessed

Run what you want, but our old (and new) Eaton Posi's are designed for use with conventional lube with the 4 oz. bottle of GM friction modifier. Eaton recommends against synthetics. The folks at Eaton say conventional gear lube "performs best" vs. synthetics.

More here:

http://www.eaton.com/Eaton/ProductsS...ials/index.htm

No doubt synthetics have their applications but not here. Being more slippery, synthetics could reduce posi-traction effectiveness.

SH-60B

It also says a friction modifier additive is not needed. Contrary to all conventional wisdom. I wonder why.

bashcraft

What do you think the friction modifier does?

jb78L-82

Correct...you add the friction modifier to make the conventional gear oil more slippery which is why you don't need it with a synthetic..

Kubs

Many of the Eaton locking type differentials use a mechanical lock instead of clutches. Due to this, you NEED friction for the interferance locks to lock. Friction modifier and synthetics are too slippery. Standard posi diffs with clutches need the modified and perform better with synthetics.

a71vette

I ended up needing 2 bottles of friction modifier, on top of running synthetic. Just had a slight whine and chatter on tight turns even after the first bottle, but now it's nice and quiet and still grabs like it should.