[firebirdcrazy1959]

Ok to use it in a 37 year old rear?Posi,i have a bottle of GM additive ...

[Formulajones]

Use a GL4 rated regular gear oil. I believe all the clutch style Eatons do not recommend any type of synthetic fluid.

[PunchT37]

I agree with formula on the synthetic but, I would go GL5 rated 80W90 with your GM additive.

[ta-kid]

I just had my 75 rear all rebuilt with a new Eaton Posi and the shop who did it,(that's all They Do is rear ends ) said not to use the synthetic as the clutches do not like it.He also prefers the ford additive over the GM.Who Knew.I had always used synthetic in all my previous corporate rear posi's with no Problem.

[77 TRASHCAN]

I've heard WAY too many negatives about the ford additive. I know the GM additive works!!!

[firebirdcrazy1959]

ok,I have 3 qts of GM lube from when I was a tech way back .

[TedRamAirII]

From EATON.

Q: What kind of oil should I use? Can I use synthetics? Do I need a friction additive/modifier?
A: Eaton Posi units perform best when using GL4 (or better) mineral/petroleum based gear oil. A four ounce bottle of friction additive/modifier is also necessary for optimum performance.

[mgarblik]

As Eaton says, GL-4 or better mineral based. GL-5 is better suited for hypoid gear sets which produce higher tooth pressure and more sliding friction between the ring and pinion teeth. Conversely, GL-5 is not recommended in manual transmissions, which don't exhibit this type of mesh. Also, even without additive, almost all GL-5 lube has some friction modifier in the lube for posi-units. This is a common cause of gear clash when shifting a manual trans fast. For your old rear axle, mineral based GL-5 80w-90 or straight 90 wt. with the GM additive would be my choice.

[dataway]

I assume the Eaton "Lockers" are different? I know the one in my late model truck uses Synthetic gear oil. The Lockers don't have clutches right?

[64speed]

Originally Posted by dataway

I assume the Eaton "Lockers" are different? I know the one in my late model truck uses Synthetic gear oil. The Lockers don't have clutches right?

No the G80 Locker in the GM trucks is a helical gear posi. So is the Tru Trac like the one I got for my 9 inch. I still didnt use synthetic though I used Lucas

[Formulajones]

Some style lockers still use a very small clutch pack, but it's nothing like the old Eatons. Other lockers use a worm gear mechanism. Synthetics have been used in the lockers without issues.

[oldblueponcho]

Originally Posted by ta-kid

I just had my 75 rear all rebuilt with a new Eaton Posi and the shop who did it,(that's all They Do is rear ends ) said not to use the synthetic as the clutches do not like it.He also prefers the ford additive over the GM.Who Knew.I had always used synthetic in all my previous corporate rear posi's with no Problem.

If it's I.W.E. you're talking about, they told me the same thing.

[Goatracer1]

I have use synthetic Mopar fluid in my Detroit Locker for over 20 years with no ill effects.

[PunchT37]

Originally Posted by dataway

I assume the Eaton "Lockers" are different? I know the one in my late model truck uses Synthetic gear oil. The Lockers don't have clutches right?

You can see here that the governor lock style posi still uses a clutch pack;


https://www.gmpartsgiant.com/parts-l...ferential.html

BUT, these newer units can use synthetic oils. I have 2 of these monsters here...