M-21 GM Transmission Lubricant

[Paul E.]

Can anyone help me with the correct GM part number for the lubricant for a 1979 M-21 transmission? My local Chevy dealer states that their database don't go back as far as 1979. They think it is the same lubricant that goes in the rear axle but are not sure. I would prefer to use the GM lubricant. Thanks!!!

[Gary R.]

Re: M-21 GM Transmission Lubricant

I don't have the GM part number but all I use in Muncie and Super T-10's is Lucas 90 wt gear oil. Any quality 90 wt will work.

[Jim T.]

Re: M-21 GM Transmission Lubricant

Can anyone help me with the correct GM part number for the lubricant for a 1979 M-21 transmission? My local Chevy dealer states that their database don't go back as far as 1979. They think it is the same lubricant that goes in the rear axle but are not sure. I would prefer to use the GM lubricant. Thanks!!!

Have you checked your owners manual? My 70 owners manual says to lubricate with SAE80 or SAE 80-90 Multi-purpose Gear Lubricant meeting requirements of U.S. Ordnance Spec. MIL-L-2105B.

[Edward J.]

Re: M-21 GM Transmission Lubricant

Paul, Regular gear oil 80w90 is what is in there, Today there are alot of choices, the stuff in GM bottles is just a re-bottle from some company maybe penzoil , mobil. any way just use a top quality name brand, theres even a lot of syth. gear oils to choose from.

[Joe L.]

Re: M-21 GM Transmission Lubricant

Can anyone help me with the correct GM part number for the lubricant for a 1979 M-21 transmission? My local Chevy dealer states that their database don't go back as far as 1979. They think it is the same lubricant that goes in the rear axle but are not sure. I would prefer to use the GM lubricant. Thanks!!!

Paul-----


There are several GM lubricants you can use:

GM #12345349 is the current transmission lubricant of approximate 80W. It is very high quality and a partially synthetic blend.

GM #12346190 is the current full synthetic transmission lubricant of approximate 80W. This is also a very high quality lubricant.

GM #89021671 is the current 80-90W lubricant used for the differential having replaced the old GM #1052271. I think it can also be used for transmissions.

[Gary R.]

Re: M-21 GM Transmission Lubricant

I pulled this back up because I was lax in my statement about using any quality gear oil in the Muncie and BW ST-10's with yellow metal syncros.

For years I have to admit I never picked up on the GL rating change on gear oils. A fellow NCRS buddy called me the other day to ask what I use and my standard answer was the same as I use for diff's Lucas 90 wt or 85-140 gear oil. Well as many of you know the Sulfur additives used in the GL5 are supposed to attack the yellow metal and cause issue with them. I don't really know how long that takes,if at all, but it's enough of a concern that I looked deeper into it and it does seem to be an issue.

I spoke with techs from Lucas and Pennzoil yesterday to see what they had to say, and both agreed to use only a GL4 in the old trans. Also they were clear not to use a synthetic GL4 as it could cause the stock seals to swell and attack the RTV sealant used. Of course I was not pleased to hear of this and contacted those I rebuilt 4 speeds for to inform them of this potential problem. Lucas said to mix thier GL5 90 wt 50-50% with their oil stabilizer but I'd rather just remove it and replace it from my Muncies and ST10's. The ST-10's in my son's car also have the Lucas GL5 in it but he tends to blow up the trans after a few track runs anyway so that is less a concern for me! LOL

Looks like CRC makes "sta-lube" in a true 85w90 GL4 rating and FORD also sells it. I'll probably take a look in the area to see what is available and use one of those if I can't find anything. I have not seen any evidence of this happening but I don't want to chance something as simple as a oil change to resolve a potential problem.

Anyone else out there have similar concerns? Since there is no yellow metal in a diff I'll continue to use the 85w140 Lucas in the diff's.

[Duke W.]

Re: M-21 GM Transmission Lubricant

Go to any auto parts store and buy two quarts of SAE 80W-90, API GL-5 gear oil. This is the modern equivalent of Mil-L-2105 that was original factory fill back in the fifties and sixties. (About ten years ago Mil-L-2105 was cancelled and the military now uses the commercial spec.) Most BW and Muncie four-speeds need about 2.5 pints.

As with engine oil, BRAND IS NOT IMPORTANT! Just buy a product that is clearly labeled with the above referenced API spec and SAE viscosity range.

There are stories all over the internet that GL-5 "corrodes yellow metal".

Does ANYONE on this forum have actual evidence that Mil-L-2105 or SAE 80W-90 GL-5 actually caused synchronizer corrosion in a Corvette manual transmission in the last 58 years.

Come on! Let's see all the evidence.

Evidence is not "I heard...." Evidence is actual eyewitness accounts of observed synchronizer "corrosion", preferably with photos of the damage.

Duke

[Gary R.]

Re: M-21 GM Transmission Lubricant

Hi Duke
I have to agree I haven't seen any damage from the oil and was a little hesitant to act on this until I spoke to the Techs from both Lucas and Pennzoil/Shell. I figured they would have put it to bed as a myth but they did not and indeed recommended not to use GL5, that was good enough for me since they should know their product better then I would.

[Duke W.]

Re: M-21 GM Transmission Lubricant

"Knowing their product" has nothing to do with this question. What counts are the guys who designed the transmissions and wrote the lubricant specifications.

Duke