I have a 69 with 3:36 rear end. It developed a little chatter this spring so I changed the oil in it with Valvoline limited slip gear oil. The chatter got much worse. So the question is, did I use oil that isn't comparable and should I change the oil to GM oil?
Thanks for the help. Chuck

 

use this
also note now Gary recommends 2 bottles of additive..works perfect n mine

 

GR8 post as usual interpon and tx for the info..
Just curious to know what is recommended for the stock rear for my 78???
Thinking about having my shop R&R the fluid in mine just because I don't think it has been changed since I'v had it about 10 yrs..And BTW there is no chatter!!!

JJ78

 

Probably same as 79?

 

Boy you pulled that one from history. I would just like to revise it a little. I now use 2 bottles of the GM additive as I believe the chemistry changed back around that time. There are other posi additives on the market as well, I have not used them to say one way or the other.

I still use Lucas Gear oil, non-synthetic. I received back a diff I built about 10 years ago that was filled with the Lucas and never installed. It just sat on the floor all that time. When I removed the cover to take a look I was very surprised and pleased to see it looked as good as the day I built it. There was no surface rust on the exposed metal because the Lucas oil still had a film covering it. I use the 85-140 grade, they also have a 80/90 wt that can be used.

The 70-79 axles (yokes mentioned above) should be checked, most were worn by 40-50k miles. As for replacing them that is another story.

The posi case history
63-64- Dana not very good
65-68 Eaton, 1st design, better than the Eaton, prone to cracking. 10-18 spiders, solid clutches
69-71- Eaton, 2nd design, better than previous ones, still can crack. 10-18 spiders, solid clutches
72-79 Eaton- 3rd design, the best of the run. Dual OD case, saw tooth seam, 10-17 spiders, junk snowflakes
80-82- Dana, the whole diff changed, not just the posi. The design was for cutting weight and since most of the cars were low power, automatics the smaller internal parts held up. They were not without issues such as cap fit, clutch retainer damage, and the same axle issues as previous years.

 

Gary i had changed the rear oil on my 73 used syn. and soon when turning it sounded REAL bad read your post and did as you suggested and now it is SMOOTH SMOOTH SMOOTH !!!!

 

Glad to hear it worked out for you. The synthetic is too slippery and doesn't work with the clutches, as you found out. I recall changing the gear oil in my 69 40 years ago with a name brand, all in one, super gear oil. It was lousy and I had the same issue you experienced, drained it out, went to regular gear oil with one bottle of the old GM additive and never had a problem.

Now if I can only convince people the stock steering boxes are not as bad as they think, kinda the same thing.

 

Gary you post so many very good articles and would like to say THANKS.