Help! rear axle fluid and posi fluid

[tomgok]

I want to change the rear end gear oil in my 76 C3, I get varying answers on gear oil type and amount of posi fluid. I would like any assistance in this task. Thanks

 

[Hib Halverson]

There is no such thing as "posi fluid" nowadays.

You need an SAE90, 75W90 or 85W90 gear oil meeting the GL5 specification and compatible with a limited slip differential.

 

[GTR1999]

I use and recommend using Lucas 85-140 gear oil, non synthetic. 1-2 bottles of Gm limited slip (posi) additive. Others have had good results with other oils but these are what I use and have it in street and strip vette diff's.

 

[Mac]

Is this the gack in question? Former made of whale oil... at least according to urban legend...

-Mac

 

[GTR1999]

Hey Mac
yes that is the additive, looks like they changed the bottle configuration but still 4 oz. I still have the old bottles in stock. They haven't made it from whale oil in about 45 years I think.

The # changed as well, used to be 1052358, now it's 88900330 about $13 a bottle

 

[Mac]

Hey Mac
yes that is the additive, looks like they changed the bottle configuration but still 4 oz. I still have the old bottles in stock. They haven't made it from whale oil in about 45 years I think.

The # changed as well, used to be 1052358, now it's 88900330 about $13 a bottle

When I was first driving, I was trying to deal with a noise in the rear diff of my 1971 GMC Sprint (similar model to El Camino) and the rather ancient mechanic up the road recommended I get a can of whale oil. The guys at the local auto supply had a great laugh at my expense.

-Mac

 

[Vettehead Mikey]

When I was first driving, I was trying to deal with a noise in the rear diff of my 1971 GMC Sprint (similar model to El Camino) and the rather ancient mechanic up the road recommended I get a can of whale oil. The guys at the local auto supply had a great laugh at my expense.

-Mac

Joke's on them, it originally was made from whale oil- way before our time though. They apparently changed it to the present formula because the original didn't stink bad enough.

 

[Hib Halverson]

Starting in the 1950s when GM introduced "Positraction" limited slip differentials, GM sold a product called "Positraction Fluid" which was a gear oil fortified with a whale-oil-based additive. That additive served as a friction modifier and was in the lube as a way to prevent plate chatter in clutch-type limited slip differentials, which were the only type of LSD available in the "old" days.

In the late 1970s, whale oil was eliminated as a result of world-wide concern for a decreasing whale population.

Today, some lubricants work well, as-is, in a clutch-type limited slip, but others need a "friction modifier" to modify the friction between the clutches just slightly such that significant chatter does not occur. GM and other car companies now sell small bottles of friction modifier that can be added in the field to eliminate a chatter problem.

Gary...I noted your post about using 1-2 bottles of FM in Lucas 90-140 petroleum-based gear lubricant.

1) Why is so much needed?
2) Do you recommend that particular petro-based lube in rear axles for which the manufacturer recommends a synthetic?

 

[Mac]

Joke's on them, it originally was made from whale oil- way before our time though. They apparently changed it to the present formula because the original didn't stink bad enough.

It was in the 80s and the whale oil had long since been eliminated so when I walked in like a goober, asking for whale oil, the auto shop supply geeks had a great laugh. They were about the same age as me at the time (young) so full of piss and vinegar, knowing little but thinking they knew it all... same as me... Eventually, as I explained what I was trying to address, they figured out to get me an expensive little bottle of additive.

-Mac

 

[killain]

Problem with Differential oil change?

Starting in the 1950s when GM introduced "Positraction" limited slip differentials, GM sold a product called "Positraction Fluid" which was a gear oil fortified with a whale-oil-based additive. That additive served as a friction modifier and was in the lube as a way to prevent plate chatter in clutch-type limited slip differentials, which were the only type of LSD available in the "old" days.

In the late 1970s, whale oil was eliminated as a result of world-wide concern for a decreasing whale population.

Today, some lubricants work well, as-is, in a clutch-type limited slip, but others need a "friction modifier" to modify the friction between the clutches just slightly such that significant chatter does not occur. GM and other car companies now sell small bottles of friction modifier that can be added in the field to eliminate a chatter problem.

Gary...I noted your post about using 1-2 bottles of FM in Lucas 90-140 petroleum-based gear lubricant.

1) Why is so much needed?
2) Do you recommend that particular petro-based lube in rear axles for which the manufacturer recommends a synthetic?

Hib, is there any sense in using a 'Synthetic' gear oil in a old 86 corvette/ or any GM product ? Getting ready to do this job, I have read that most of the 'Synthetic gear oils already have "Friction modifiers" already in the 75W/90 fluid, in which you don't use any of the friction modifier unless you experience chatter in the differential. At that point you need to with draw some of the gear oil and add some of the additive or friction modifier. This seems to me as something akin to trial and error or seat of your pants kind of chemical experimentation. I think it's more of a PITA procedure. What do you think ? Is synthetic gear oil worth the trouble ? Thank you ! :ugh

 

[Hib Halverson]

Hib, is there any sense in using a 'Synthetic' gear oil in a old 86 corvette/ or any GM product ? Getting ready to do this job, I have read that most of the 'Synthetic gear oils already have "Friction modifiers" already in the 75W/90 fluid, in which you don't use any of the friction modifier unless you experience chatter in the differential. At that point you need to with draw some of the gear oil and add some of the additive or friction modifier. This seems to me as something akin to trial and error or seat of your pants kind of chemical experimentation. I think it's more of a PITA procedure. What do you think ? Is synthetic gear oil worth the trouble ? Thank you ! :ugh

As to whether or not to use a synthetic gear lube in an 86 depends on how you use the car.

Some drive axle lubricants have friction modifiers and some do not.

Also with FMs, not all friction modifier additives are compatible with all gear lubes. For example, the "GM additive" is not good to use with some synthetic gear lubes.

The need for friction modifiers will vary on the condition of the limited slip differential, how the car is driven and whether or not the driver can tolerate some noise.

If I put an FM in a gear lubricant, I only use enough to "quiet" the axle such that in slow speed, tight turns with the axle lube at operating temperature, I hear a subdued squeal from the diff. Some people like to use enough additive to make the limited slip silent, but I choose not to do that. Clutch type limited slips work on friction. Friction modifiers decrease friction so there this happy medium point where you balance differential silence vs limited slip action. I'll take a little squeal and better limited slip action.

That said, if the limited slip clutches are chattering, I'll use a little FM but, once I get to an ounce and a half or so of the stuff and the diff is still chattering or squealing loudly, I'll repair the diff.

 

[db2]

FWIW...I use this.

 

[GTR1999]

Hey Hib
Sorry I didn't see this for awhile. I now use 2 bottles of the GM LS additive because I believe it has changed over the past year or two. The part # & bottle have but I never needed to use more then one bottle with a tuned posi until last year. I tuned a posi and had a chatter from it, not typical with a tuned posi. Before tearing down the posi I added a 2nd bottle of the new additive and that solved the problem. That was the one and only time I had tuned posi chatter and none since recommending the 2 bottles of the new GM additive.

 

[Hib Halverson]

FWIW...I use this.View attachment 18428View attachment 18429

:L
Not much of an endorsement as you're an Amzoil dealer.

 

[Hib Halverson]

Hey Hib
Sorry I didn't see this for awhile. I now use 2 bottles of the GM LS additive because I believe it has changed over the past year or two. The part # & bottle have but I never needed to use more then one bottle with a tuned posi until last year. I tuned a posi and had a chatter from it, not typical with a tuned posi. Before tearing down the posi I added a 2nd bottle of the new additive and that solved the problem. That was the one and only time I had tuned posi chatter and none since recommending the 2 bottles of the new GM additive.

Obviously, you do far more rear axle work than I. Plus, you're not always in a position to road test the vehicle for which you build the axle and you're often dealing with customers who believe the diff not only must be chatter free but also be silent. No doubt, the suggestion of using 2 bottles of that additive may stem partly from that, but I still maintain that the best way to adjust the amount of FM in a clutch-type limited slip is by a little testing. Also, I believe that the "GM additive" works best with gear lubes having a similar base stock and blending as the rear axle lubricant GM currently uses which, the last time I checked, was a blend of petroleum and synthetic base stocks supplied by a Canadian lubricant maker..."PetroCanada" I think.

Based on your recent experience, I'll speculate that GM's Dexron LS may require more additive than some full synthetics. For example with the Red Line drive axle lubricant I use, I've never needed anywhere close to two bottles of friction modifier. The most I've use is 1.75 ounces. That was in my C4 and that got me to the point where, with the rear axle lubricant at operating temperature, in a parking lot doing tight turns in both directions at slow speed I have no chatter but a subdued squeal coming from the diff. I've never needed any friction modifier in my C5 or my C6, both of which use the same Red Line drive axle lube.

 

[Mac]

Also, I believe that the "GM additive" works best with gear lubes having a similar base stock and blending as the rear axle lubricant GM currently uses which, the last time I checked, was a blend of petroleum and synthetic base stocks supplied by a Canadian lubricant maker..."PetroCanada" I think.

PetroCanada was bought up in 2009 by Sunoco. In Canada, they rebranded all of the Sunoco stations to PetroCanada stations.

Mac

 

[68Roadster]

This has been an interesting read since I am just about to do the fluid change again. I have always used the GM lube and additive but I continue to have the "noise" from the rear end.
This past weekend I took the '68 out for a drive which included approx. 20-25 minutes of highway driving @ around 70mph. Once off the highway and after coming to a stop I was greeted with the chattering noise as I pulled a right turn and headed down the street. I have had this issue ever since I bought the car in 1996 but it never sounded as bad as what I heard the other day. I thought a wheel was gonna fall off or I would see the diff laying in the street behind me. I can feel this in the seat of my pants when it happens and I'm getting a bit tired of it. There are times it even feels like the back end is swaying during a turn. The car has 107k miles, has never had any diff work done that I can tell (I'm 2nd owner) and I'm wondering if a rebuild is in order? I'm gonna try a fluid change first and if that doesn't quiet this down I guess I'll opt for the overhaul.

Thanks for the info....

 

[GTR1999]

Could be tight clutches. You can try sucking out the old nasty oil, adding (2) bottles of fresh Gm additive and topping off with the LUCAS 85-140 and doing figure 8's in a parking lot. If that doesn't work you can test a different additive but you might be looking at diff/posi work. I'm building a posi now in fact.

 

[legacy76]

Guys, please excuse my jump.

What is involved in lowering the gears? Have stock now.

Thanks.




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[GTR1999]

Changing gears is pretty involved, I have threads online if you do a search you should be able to find them.