rear end gear oil GL6?

[Keith B.]

is GL6 compatible with mid year rears?

[Duke W.]

Re: rear end gear oil GL6?

GL-6 is an obsolete spec that was originally designed for "high offset" pinion hypoid axles.

The GM hypoid axle and manual transmission spec going back to at least the forties was Mil-L-2105D. This spec was superseded by the commercial API spec, SAE 80W-90 GL-5, in the seventies, and the military eventually canceled the mil-spec and substituted the API spec.

Duke

[Keith B.]

Re: rear end gear oil GL6?

I have no clue what a "high offset" pinion hypoid axles are. but the person who did the rerar diff said to use this
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[Duke W.]

Re: rear end gear oil GL6?

I prefer to stay with the current version of the original mil-spec, namely SAE 80W-90 GL-5. Brand makes no difference as long as it meets the API spec.

The package does not contain the API "donut", so I suspect it is not API certified, and it doesn't meet the GM recommended viscosity spec.

Duke

[Paul H.]

Re: rear end gear oil GL6?

I was just exchanging e-mails with Gary Ramedei on Saturday regarding the current state of affairs and what is best for our old Vette rear diffs. He likes Lucas 85/140 or 80/90 GL5 with 2 bottles of posi lube. He advised that the formula is a bit different with the posi lube than the old days and now recommends 2 bottles. I can tell you that I have over 300,000 miles on my 65 rear diff and it's never been out of the car. Always used 80/90 gear weight with 1, 4 oz bottle of posi lube. Regular changes, about every 4 years. I drive ALOT.

[Bill B.]

Re: rear end gear oil GL6?

Paul, I also second Gary's advice! I had severe groaning when turning the corner backing out of my driveway. I had switched to the GM synthetic differential fluid with two bottles of posi lube. After that, my groaning issue developed slowing over time and use.

Gary recommended draining (I used a sucker with a long thin flexible hose) the synthetic fluid out and replacing with the Lucas 85/140 GL-5 with 2 bottles of the GM additive. Instant cure! No clunking or groaning.

Gary definitely knows differentials!

[Gary R.]

Re: rear end gear oil GL6?

Thanks guys.
One thing I often forget to mention is that there are a lot of fancy oils out there today and I would not use a synthetic oil in a vette diff. Now I know, many will say they use the fancy stuff with all additives and juice in them and that is not my point. The point is some of the synthetic oils break down RTV and cause leaks. I used to know a very knowledgeable tech at Permatex who told me years ago of this problem and it led to them coming out with a specific RTV for differentials, I call it the green cap RTV as that is how it is packaged. Again, I am sure that there are some out there who won't accept that reasoning and that's ok. I stick with gear oil.