Rearend gear oil suggestions

[Bob H.]

I should know this but am confused. I was always told not use synthetic oil but conventional in the rear but it looks like the GM recommended oil 10-4108 is synthetic. Is this the correct to use along with the limited slip or are there other suggestions on what to use these days (72)

[Leif A.]

Re: Rearend gear oil suggestions

Bob,
I like PennGrade-1 Classic GL-5 gear oil 80w-90. Add two bottles of LSA, of your choice.

[Mike T.]

Re: Rearend gear oil suggestions

Bob - Couldn't agree more with Leif, go with the Gl-4 rated 80w-90 gear oil and a couple bottles of the posi additive and you'll be good to go.

[Paul H.]

Re: Rearend gear oil suggestions

X3. Agree. My proof that it works:Had my 65 53 years and driven it over 300,000 miles. Original rear end has never been rebuilt. I don't baby it either. I'm just religious about periodic fluid changes. Always used GL-4 80/90 gear oil and 1 (or 2 now) 4 oz of GM posi lube. Put the lube in first before you fill. Then go do your figure 8's.

[Bob H.]

Re: Rearend gear oil suggestions

Thanks. Will follow your advice!

[Duke W.]

Re: Rearend gear oil suggestions

GL-4 IS NOT SUITABLE FOR HYPOID GEAR AXLES.

It might be okay for transmissions because they don't have the highly loaded hypoid gear teeth that axle ring and pinions have.

Buy SAE 80W-90 GL-5. Any brand is okay including house brands as long as they have the API "donut" with the underlined spec I quoted.

If you look at C1 and C2 owner's manuals and service literature the gear oil spec for BOTH manual transmissions and axles is Mil-L-2105 with various suffixes the last of which I think was D. This mil spec was cancelled in the seventies and replaced with the commercial API SAE 80W-90 GL-5. Many claim that GL-4 is okay for our old manual transmissions, but GM said Mil-L-2105 and the modern equivalent is GL-5.

"Boutique" oil vendors claim that GL-5 corrodes "yellow metals". There is evidence that GL-5 may corrode "yellow metals", but only at higher operating temperatures than an automotive transmission is ever going to see, so it's basically a myth as far as car transmissions are concerned.

I've talked to guys who have overhauled more vintage manual transmissions that they can remember and they claim to have never seen "corrosion" on brass synchronizer components. Wear, you bet, but not corrosion.

The friction modifier used by all manufacturers that offer clutch type LSDs can be purchased at NAPA, NCB 4285V, 4 0z. bottle and any parts store should be able to cross reference this to whatever brand they carry.

You don't have to buy the friction modifier from GM.

Duke

[Leif A.]

Re: Rearend gear oil suggestions

Duke,
Thanks for catching my "fat fingered" typo. I meant to put GL5, not GL4 for the differential. GL4 is fine for the transmission, though. I fixed my original post.

[Bob H.]

Re: Rearend gear oil suggestions

thanks. I was about to remove the Valvoline 80-90 GL5 I just put in the rear with gm posi fluid (2). Sounds like that's ok. I will put the Penn GL4 I bought in the transmission

[Mike T.]

Re: Rearend gear oil suggestions

Well, you guys got me wondering whether I used GL-4 or GL-5 in the differentials of my Midyears and I remembered reading/hearing about the supposed corrosion issues with GL-5 lubricants so I went out to the garage to see what I had used since it's been a while and found that I had used GL-4 Sta-Lube. No issues to report and it does indicate it is for Hypoid Gear use. My apologies for the sideways pics, it seems to happen about half the time I try to post pics.
IMG_1307.jpgIMG_1309.jpg

[Paul H.]

Re: Rearend gear oil suggestions

Just looked at my Lucas SAE 80W-90 Gear Oil bottle in the garage. Bottle says "exceeds service level API GL5, API MT-1, SAE J2360(MIL-PRF-2105E), Mack GO-J, PG-2 Limited slip.'
Yea, looks like GL-5 is the correct spec now.
Bob, sorry for any confusion I may have caused you. Not sure what I was using back in the early 70's. It was 80/90. It worked. GL-5 IT IS, NOW.

[Bob H.]

Re: Rearend gear oil suggestions

thanks. I'm glad I am not the only one a bit confused. Corvette supply houses are selling silicone and that's what got me confused. I'm confused daily anyway!

[Mike T.]

Re: Rearend gear oil suggestions

Okay, bit of an update. I thought I had used GL-4 Gear Oil in my Maroon L78 Roadster because I ran across an opened Sta-Lube container. That was a couple days ago. Since then, I started thinking I should have seen the pump attachment since that's what I use when filling a diff (or trans) when they are installed in the car. Looked around a bit more since I'm in the middle of changing the diff fluid in my 66 L72 Roadster and 'voila', found IMG_1323.jpgthe pump and it's still attached to a gallon container of GL-5 diff lube. Also ran across a few quart bottles of 85-140 GL-5 too.