# Rear Gear Oil Recommendations



## Josh.AZ.GTO (Jul 11, 2009)

I just had my rear end torn down and reassembled and it currently is sitting with no oil added from the shop. I will need to add gear oil but was wondering what you all might recommended for refilling it. 

Thank you.


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## geeteeohguy (Feb 2, 2008)

Depends if it's a limited slip or not. In mine, (LSD), I use Gl-4 85-90W with an LSD additive. Sometimes you can find limited slip gear oil with the additive already in it, but I add a bottle just to be sure. No synthetic recommended in an LSD...it's "too" slick. If a regular open rear end, any Gl-3-GL-4 85/90W will suffice.


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## BearGFR (Aug 25, 2008)

If perchance you have some sort of aftermarket internals, then please DO seek the recommendation from the various menufacturers involved. For instance, I'm running a Wavetrac differential in my car that has a lifetime warranty --- unless I use a non-approved lube. For an open (non-limited slip) diff any good quality lube will work fine, either dino or synthetic as is your preference. I'd recommend using something that meets API GL-5 specs. 

"API Category GL-5 designates the type of service characteristic of gears, particularly hypoids in automotive axles under high-speed and/or low-speed, high-torque conditions. Lubricants qualified under U.S. Military specification MIL-L-2105D (formerly MIL-L-2015C), MIL-PRF-2105E and SAE J2360 satisfy the requirements of the API GL-5 service designation."

Things get "sticky" when your talking limited-slip axles. My Wavetrac for example states explicitly do NOT add any "limited slip" additive or use a lube that states it's formulated for limited-slip axles because it works differently from a traditional clutch or cone type axle, and the additive will make it NOT work. - and will void the warranty.

Bear


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## olde-goat (Feb 1, 2009)

If we are talking about a GM Posi, GM dealer parts counters still sells the Diff Friction Modifier additive in small 4 oz bottles. PT #992694. Definitely no on the synthetic.


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## Josh.AZ.GTO (Jul 11, 2009)

I have the original 3.31 gear BOP 12 Bolt. Shop said everything looked really well and only recommended replacing wheel bearings. So it is pretty stock the way it was in 1970.


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## ALKYGTO (Mar 29, 2010)

Josh.AZ.GTO said:


> I have the original 3.31 gear BOP 12 Bolt. Shop said everything looked really well and only recommended replacing wheel bearings. So it is pretty stock the way it was in 1970.


Josh, I believe your car if an original 455 GTO should have the Chevy 12 bolt rear, not the BOP (Olds) 12 bolt. Chevy type has an "eyebrow" across the cover while the Olds is smooth.


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## geeteeohguy (Feb 2, 2008)

At this point, we don't even know if he has a limited slip rear end or not. It's anybody's guess.


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## Josh.AZ.GTO (Jul 11, 2009)

geeteeohguy said:


> At this point, we don't even know if he has a limited slip rear end or not. It's anybody's guess.


Not limited slip. I was told it wouldn't need additive by the shop when I picked it up.


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## Josh.AZ.GTO (Jul 11, 2009)

ALKYGTO said:


> Josh, I believe your car if an original 455 GTO should have the Chevy 12 bolt rear, not the BOP (Olds) 12 bolt. Chevy type has an "eyebrow" across the cover while the Olds is smooth.


It does have the "eyebrow" you speak of and is the original rear end to the car, and yes it was a 455. I was under the impression that GM 12 bolts had different internals pending it a Pontiac or Chevrolet. I have some pictures posted in my restoration thread of the rear end as of recently back from the shop.


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## ALKYGTO (Mar 29, 2010)

I'd just do some 90W for now in either conventional or synthetic up to you and your budget.


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