Limited slip gear oil additive.

[petes74ttop]

I'm going to change the differential oil, can I use Trans X additive, from the auto parts store, or do I have to get GM's additive? Is 8 oz of additive correct and what weight oil should I top it up with? PG.

 

[JohnZ]

Forget the aftermarket stuff. You want to suck out all the old lube with a suction gun, add a bottle of the GM additive, then fill it with GM Diff Lube or a brand-name 75W90 or 80W90 GL-5 diff lube and do some low-speed figure-8's in a parking lot to get the fresh lube and additive inbetween the Posi clutch plates. That's what it was designed to work with - you don't need any of the hot-rod whizbang miracle race lubes.

 

[petes74ttop]

Thanks for the info and the quick reply, PG.

 

[6880 Mike]

...Forget the aftermarket stuff....GM additive...GM Diff Lube...

I agree. Stick with the stock GM stuff.

 

[Phill]

Do you add one or two of the gm additive. i think ive heard or read it was two.

 

[Hib Halverson]

If the diff. is in good condition and you're using the proper rear axle lubricant, about 2 oz of "additive" (it's really a "friction modifier") should be enough. There are two ways to add the stuff. The easy way is to just add 2 oz and see what happens. If it's still making noise add more bottles of additive until the noise stops.

However, because a friction modifier works to eliminate noise and chattering from clutch-type limited slips by degrading limited slip action, the best way is to only add enough friction modifier to eliminate chatter and almost eliminate noise. That way you get the best compromise between limited slip action and no chatter.

What I do is: after changing the lubricant, I add 1 oz of friction modifier. Then I go for a road test. I start with a 10 mile run on the highway to heat the rear axle lube to operating temp. Then I go find a parking lot and drive in tight circles at low speed once in each direction. If I feel chatter or hear loud squealing from the diff, I go back to the shop and add another ounce. Then I road test again. If the chatter is gone and all that remains is a subdued squeal in the tightest turns, I don't add any more additive. If the chatter still exists or the squealing is really loud, I add a third ounce. Then, I road test again. If chatter or noise persists after adding three ounces, if repair is not an option, I'll either go for a fourth ounce. If repair is possible, I'll consider rebuilding the diff. because generally, a diff that needs a great amount of limited slip additive to stop chatter or quiet the squeal may need to have the limited slip unit repaired.

Most rear axles in C3, 4 or 5 that I've worked on respond well to 2-3 oz of additive. But, any more than three and, again, I figure the limited slip probably needs work.

Lastly, be careful about what additive you use. If you're putting a petroleum-based gear lubricant in your axle, use the limited slip additive sold by GM dealers.

If you use an aftermarket synthetic gear lube (ie: Mobil 1 or Red Line) you may need to use a friction modifier other than what GM sells for best results. In the case of synthetic lubes, consult the lubricant refiner before buying.

 

[GTR1999]

When I set up posi's I don't use the springs. I use 4 oz of the GM additive and never have any hammering. Now if you're running a stock posi with the springs, especially a new Eaton with those giant yellow springs, then I use 8 oz of additive. The clutches are shimmed to set the backlash in the spider gears. If they are too loose they will chatter and possibly lockup. Typically stock eaton units have a backlash range off 000-008" many times run wider then this. Rebuild ones can be all over as well.
Try the additive, better yet suck out the oil( I tap a drain hole in the housings) and add Lucas 85-140 gear oil and 4 oz of the additive. That should resolve the problem, if not then the posi should be removed and tuned.

 

[C3forME]

Forget the aftermarket stuff. You want to suck out all the old lube with a suction gun, add a bottle of the GM additive, then fill it with GM Diff Lube or a brand-name 75W90 or 80W90 GL-5 diff lube and do some low-speed figure-8's in a parking lot to get the fresh lube and additive inbetween the Posi clutch plates. That's what it was designed to work with - you don't need any of the hot-rod whizbang miracle race lubes.

I agree with John. I just did mine this exact way and it worked like a charm. I bought a suction gun and 2 bottles of Valvoline 80w90 gear oil from Napa and a 4oz bottle of GM limited slip additive from my local Chevy dealer. The total job took about a half hour and then I was out doing figure eights.

 

[petes74ttop]

Hi everybody and thanks for the replies. Heres some background. I bought my 74 T Top in Nov of 2000 ( with 50,000 miles on it ) I replaced the motor with a Universal 350 ( 10067353 ) and at the same time had the TH400 rebuilt. The rear end has stayed toped up but I now have 80,000 miles on the car so I figured I'd change the rear end oil. I never spin the rear tires and I never have gone over 4,000 RPM but I think that the differential oil has been in there for 34 years so I think it should be changed, it doesn't make any noise nor is there any other problem with it.
So I went to GM today and got the additive, so should I just put in one bottle and top it up with 80W90GL5?
While I'm at it I figured I'd change the Transmission filter and top it up. Good or not good? Thanks again for the help. PG.