Just got back from the auto store. Bought 2 bottles of Lucas Gear Oil 80-90 for my rear differential 70 Chevelle SS with 3:31 posi). It says on the bottle that the the posi additive is not necessary. Looking for thoughts on this. The guy behind the counter at Advance said it won't hurt anything to add it. I want to add it anyway. Just wanted to hear from people who may have run into this situation before. Thanks in advance. Tom.
Can you even find a name brand gear oil without the additive in it? I just don't want "too much" additive in the rear end if there is such a thing. I would like to use the GM additive since I already have it, and I am willing to get different gear oil if there is even a small concern. Thanks again. Tom.
You can find gear lube "without the additive". It is API GL-4. More likely to find it in a hardware or farm supply store. Supposedly the "additive" makes it more likely that the oil will attack copper/zinc alloys brass bronze etc. like maybe synchromesh components in a transmission. I wouldn't for instance use GL-5 in my roto-tiller because it has a bronze gear in the transmission.
In any case, some good quality API GL-5 gear lube will be fine for limited slip differentials including "Positraction". See here for some words and specs and applications.
If it was me, I'd take the Lucas and additives back, and check some Pennzoil or Quaker State or other brand name GL-5 spec and see if it covers limited-slip. If it does I'd run it, and use some additive only if I had any grabbing or chattering like I did with my '65 after 90,000 miles. I replaced the stock fill with Sta-Lube GL-5 90 wt., and its been OK since, w/o any additives.
Just my $0.02
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