Ray Y

If you run sythetic lube in the rear diff, do you still add the GM special Lubricant Additive they make for limited slip axles?

I have the additive and it doesn't say that it is not compatible with synthetic lubricants, but before I put it in I'm looking for feed back. Changed over to synthetic in the gear box last night.

Thanks in advance for the input!

larrymcgraw

If you happen to be using Amsoil synthetic lube, I would use their additive as well. If you're using Royal Purple, they probably also make an additive.

72-LT1

Royal Purple says it has the additive already in it.

Jim_Harrison

If the rear chatters or you start spinning the inside tire under power then the syn lube by itself isn't working. I gotta say that these old positracks work really good when set up correctly. I never spin the inside tire under power even on the slickest surfaces. (I use the GM lube and additive) I have seen cars with very expensive systems that don't work anywhere as good. Go drive a Mini Cooper S.

73, Dark Blue 454

The engineers at Eaton strongly recommend the use of conventional lubes,..not synthetics.

From the Eaton page:

"What kind of oil should I use? Can I use synthetics? Do I need a friction additive/modifier?
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."

More here:

http://www.eaton.com/EatonCom/Produc.../FAQ/index.htm

cm-smith

Stick with GM. Synthetic didn't work for me, even with extra additive. I thought my rear end was going to come apart. Back to GM lube with GM additive and all is well.

burners

I use Mobil One with no additive. 60,000 miles and everything is fine. I agree that it depends on the manufacturer. What brand did you use?

SpE3dF0x

Noticed were all from TEXAS!!!!!!!!!!!!! Was questioning this myself, I plan on using Mobile 1 (Syn Lube)

Ray Y

Yes, migrated to MT in 1990 from N TX area, still get back to Dallas & Austin several times a year. I love that area just north of you around Bandera.

I spotted another thread on this topic just below my original one, and it looks like there are guys running Mobil 1 both with and without the additive with success.

73, Dark Blue 454

Why in the world would not accept the Eaton engineers' advice to use conventional lubes? Eaton is the company that made (and makes) the posi units for our cars. I'm guessing the posi clutches are designed for use with the conventional lubes (and the GM friction modifier). And/or, the failure rate of posi units returned to Eaton from warranty claims, was higher using synthetics.

Perhaps you have low opinion of R&D or engineers.

Eaton posi's have probably logged a 900 gazillion trouble-free miles over the last 50 years, using the manufacurer's recommendation which calls for conventional lube.

But it's your rig,..run what you want.

WRH

Changed mine to Red Line. The clunk from reverse to drive and back stopped after change to Red Line. Red Line bumper to bumper.

Solid LT1

Years ago when I was doing a thrash getting my 4.56 gears into my 72 LT-1 I took off out of the driveway and thought I had forgotten to tighten the halfshaft bolts because of the violent snapping. I turned right back around, jacked the vette up and thoroghly inspected the rear drivetrain finding nothing loose, it then dawned on me that I had put in synthetic lube that did not contain friction modifiers for the Posi unit. I went out to my local "test track" and did some 2500RPM launches thinking if anything was going to break, that it would do it there, everything held up and off the the races I went (running 12.28@117MPH on slicks.) The differential settled down after the lube circulated throught the clutch pack and I got 4 years of good service out of my 4.56 differential before clutch pack wore to the point of needing refreshing. You rear end will tell you if you need Posi additive and synthetic will work with STEEL Eaton clutch packs, I'm not sure about Carbon Fibre ones but, I don't think much of CF clutch packs.

I'm Batman

I've been running Mobil 1 and the GM additive, no problems.

Did synthetics even exist when our LSDs were designed in the mid-60s?

spedaleden

I'm running GM synthetic with additive no problems

cm-smith

Mine was mobil 1 with a non-GM additive. I was afraid to go more than a block down the road it made so much noise and cluncked violently. Went back to GM lube and additive, and it was immediately better. Maybe it was the additive I used with mobil 1, but I'm convinced to stick with GM conventional.

Texashark

Can someone define 'GM Conventional'? From what I am getting from this...it is not synthetic....I think it is 75W90, GL-5. And requires the bottle of positrack special lube.
So would any quality GL-5 75W90, non-synthetic with the additive bottle, would probably work great.

Thoughts?

jb78L-82

Mobil 1 with no additive for 20+ years. Posi is perfect and operates better than it ever did with gm conventional lube with the gm additive. Synthetics did not exist when the Eaton diffs were designed.If the diff makes noises when you switched to a synthetic, something is wrong with your posi!

pws69

Same here for years - works great!

However, jb78L-82 makes interesting points - I may try without the GM Additive next time...

We didn't need no stinkin' synthetics...

68Thunder427

Actually it is GL-4 conventional with the LSD additive. Get mine at Napa.