[justincampbell]

So, I think I messed up. But, I thought I'd ask here before I make a mess to correct my potential mistake.
So, I just got done cleaning/painting my Firebird rear end as well as replacing the gasket on the diff cover. I put in limited slip additive and followed with the appropriate amount of Lucas gear oil https://lucasoil.com/products/gear-o...0w-90-gear-oil.
Shortly after I did that, I came in to order some more gear oil(to have on hand) and read something that either wasn't on my old label or I had merely overlooked. It says "Excellent for use in limited slip differentials". That would lead me to believe the additive was unnecessary.

So, the question is this: Am I going to eventually harm something by essentially having too much additive? Or will the posi just not work as well as it should?

Also, if this makes any difference, right now it is a stock 400 but I have been considering installing a much healthier 455.

[77 TRASHCAN]

Don’t worry about the additive. I’ve used 2 bottles of the GM stuff, nothing to report.

When I bought my TA new, the rear end started making noise, going around corners. It was still under warranty, not maybe 8000 miles. Don’t know the exact reason, they drained the oil, put in additive and new oil, fixed it perfect. I don’t use synthetic oil either...

I’m an expert at nothing, but only recommend the GM additive for a clutch type posi. I know it works. No way I’m gambling on something I know nothing about. Have read here about other products that do not work...

[leeklm]

Agreed, I too have tried a couple of other generic additives and rear end would chatter. The GM additive cleared it right up.

Your diff should be fine, at least until you install the big power motor and drag slicks...

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[justincampbell]

The additive I used was labeled "Jegs", so I have no idea who actually made it.

[leeklm]

Originally Posted by justincampbell

The additive I used was labeled "Jegs", so I have no idea who actually made it.

See how it goes. If the diff sounds smooth and no noises, you are good. The lube and additives you are using will not hurt anything, but If you start hearing or feeling chattering and almost like a clunking noise when making a turn or driving on an uneven surface (like my back lot ), drain the lube and refill with the GM additive.

I have nothing against synthetic lube but for a diff like this, you may better served using standard 80/90 with the ACDelco additive. With that said, dont drain your current fill unless the chattering starts.

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[Old Man Taylor]

The only additive I've had that worked well is the Ford additive, purchased from Ford.

[76TA462]

Just my 2 cents. I have always been advised not to use synthetic. Done a few changes and never a problem using SAE80W-90 non-synthetic and differential additive AC Delco 992694.

[geeteeohguy]

Originally Posted by Old Man Taylor

The only additive I've had that worked well is the Ford additive, purchased from Ford.

30 years ago, when I had the 8.2 rear end of mine overhauled by Larry Woltzen of California Differential, he said the same thing. Ford additive is the only thing he would use in these GM diffs. He also was published in High Performance Pontiac magazine back in the '90's with an article about overhauling the 'non rebuildable' cone type limited slip in these cars. My '65 rear end has been flawless since the rebuild 30 years ago. I used generic NAPA LSD additive in my other GTO with limited slip, and my Toyota Tundra, without any issues. FWIW...

[justincampbell]

I didn't realize there were such strong opinions regarding the LSD additives until I started trying to answer this question.

I assume the Jegs additive is ok as I've been using it in the posi in my '56 Chevy and haven't felt any 'clunking'(listening doesn't mean much since it just has baffles in the fenderwell headers=very loud).

[leeklm]

Yeah, some of the additives just simply do not work in the OEM style friction disc diffs. Not sure what the difference is with those additives, but they are not all created equal!

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[Schurkey]

...and don't use ANY additive in the "truck" G80 Gov-Lock "locking" differential.

[justincampbell]

Thanks guys!