shiftbyear

Is there a preferred gear lube for a 1970 "O" type rear with posi. Also what posi additive is recommended. Thanks

72455

If you get the Valvoline 75W-90 conventional it already has the additive.

Funkwagon455

I’m a proponent of synthetic oil and use it in many of my vehicles, but most advice I’ve received with regards to differentials has been any SAE approved gear Dino/oil with additive. I think the additive is available on Amazon now, probably Walmart too.😂 Perhaps Monzaz on this site can give some sound first hand advice.😄. Maybe best not to overthink it.

Fun71

I have also read that synthetic is not recommended with a clutch type posi (it may have been monzaz who posted that).

monzaz

We have used this a few times in our builds....AND has failed us every time for noise on posi units. PLEASE try to find something else...ANYTHING else.
Think about it...how much additive is put in there....Exactly YOU DO NOT KNOW!
Buy your gear oil and your additive separate...this way you KNOW the percentage mix you are using.
PLEASE everyone...Stop cutting corners in the world. This is what has doomed our country for 50 years now.
China cheaper this cheaper that... I am not saying BUY 100 dollar a quart fluid .... Just do it the way the factory would have years ago.
80/90 85/140 weight does not really matter. NON synthetic you do not need synthetic - But Friction based units like Eaton and Auburn WORK off of friction to work properly so synthetic CUTS FRICTION...and that means your cutting your posi units ability to give you what you bought it in the first place for.
Additive:
4 to 8oz... you really can not go overboard on this stuff.. but for A new unit I would almost certainly double the additive on break in. As the unit gets more miles on it 10k - 20k then just use the 4oz as required
IF you forget the flud or install the inferior Valvoline with additive already in it...and it chatters ...you will have a LONGER time trying to get it to quiet down as the additive will have to push all the inferior oil from the clutch to get the correct stuff in so that is why ...FROM THE BEGINNING , just do it right.
And as I always say.......IT IS YOUR CAR- you can do what you want...Were all just here to help get the info out there.
Jim
JD Race

monzaz

Synthetic DOES have its place - Engines after that have been broken in seems to be a good place.
EVEN rears after breaking with out FRICTION base posi units (open carrier or true trac spools locker like power traxs etc all ok with synthetic.)
Transmissions again are a touchy area... Being the thing is driven by clutch discs (AUTOMATIC) the standard maybe just fine...
It is like this.....AGAIN, If you have not had issues and have been using it...Then continue to do so.
We are just here putting out info for people to go through to help.
Our issues stem for the use of VALVOLINE... this is just fact for us.
Maybe we got a bad batch over a 5 year run of trying it...BUT we are done with that brand.
Jim
JD Race

ROCKET VAPOR

Sta-Lube offers yellow metal safe GL-4 Hypoid Gear Oil, pick the weight you desire.
I got the little bottle of friction modifier from GM.

https://www.crcindustries.com/api-gl...5w90-32-fl-oz/

olds 307 and 403

https://www.fuel.crs/wcm/connect/www...S&CVID=mje2p.N
I plan on using this in the conventional 85W140, I haul fuel for them, along with a tube of CRC posi additive in my 70S. I saw expensive Motorcraft synthetic put in a Ford truck with a posi at the dealer when apprenticing. It chattered even after throwing a whole bunch of additive in it, mostly went away. I run synthetic in my Dakota's 8.25" Trak Lok posi rear. It sees a lot of-40 driving so it gets synthetic. I may consider the 3.09 posi rear upgrade for my AWD Challenger and they all run synthetic, rear diff called for 75W85, I ran synthetic 75W90 in both. But in our old stuff with the expensive clutch posi that don't see Winter, run good conventional with extra additive.