Author Topic: M21 Correct Oil  (Read 10002 times)

miket1

  • Member
  • ***
  • Posts: 87
    • View Profile
M21 Correct Oil
« on: July 20, 2015, 04:46:38 PM »
What Oil did GM install in the M21 Transmission at the factory,  was it a GL5 oil, was it a 75w90 .

Thanks
69 Z28 Burgandy

L78 steve

  • Hero Member
  • *****
  • Posts: 797
    • View Profile
Re: M21 Correct Oil
« Reply #1 on: July 20, 2015, 05:06:23 PM »
GL5 is the grade 75w90 is the weight. You will find both on the same container. That is fine for your Muncie.
69 Z/28 Dover White. SOLD
67 SS/RS Mt. Green 1W,2LGSR,3SL,4K,5BY,07C. SOLD
70 Nova L78 Blk. Cherry,Sandalwood,M21,02B

bcmiller

  • CRG Member
  • *****
  • Posts: 4012
    • View Profile
Re: M21 Correct Oil
« Reply #2 on: July 20, 2015, 08:23:47 PM »
I always use the GL4 85w90 from NAPA. StaLube I think is the name.

Do not use GL5.

Bryon / 1968 Camaro SS 396 coupe - now old school 468 big block
1967 Camaro RS/SS 396 coupe L35/M40 - 4 generation family project
Looking for 68 Camaro with body # NOR 181016

1968RSZ28

  • Hero Member
  • *****
  • Posts: 6188
    • View Profile
Re: M21 Correct Oil
« Reply #3 on: July 20, 2015, 08:58:25 PM »
Any brand-name 75W90 or 80W90 gear lube will work fine - that's what it was designed, developed, and tested with.

Paul

bcmiller

  • CRG Member
  • *****
  • Posts: 4012
    • View Profile
Re: M21 Correct Oil
« Reply #4 on: July 20, 2015, 10:10:40 PM »
I disagree. You don't want to use GL5 or any synthetic lube in a Muncie. GL5 is not good for the brass synchros.

I know about 8 years ago crash was recommending Pennzoil 80/90 gear plus which was also GL4 back then. Not sure on that now.
« Last Edit: July 20, 2015, 10:44:50 PM by bcmiller »
Bryon / 1968 Camaro SS 396 coupe - now old school 468 big block
1967 Camaro RS/SS 396 coupe L35/M40 - 4 generation family project
Looking for 68 Camaro with body # NOR 181016

m22mike

  • Member
  • ***
  • Posts: 286
    • View Profile
Re: M21 Correct Oil
« Reply #5 on: July 21, 2015, 12:01:28 AM »
X2, no synthetic in a Muncie. Good read

 http://www.widman.biz/uploads/Transaxle_oil.pdf
X66 L78 M22 4.10 Deluxe Threads, PNT 10/10, Red Hockey stripe

miket1

  • Member
  • ***
  • Posts: 87
    • View Profile
Re: M21 Correct Oil
« Reply #6 on: July 21, 2015, 12:36:05 AM »
I completely drained my M21 to install new oil ,it looks like I need  GL4  NON Synthetic, 
 Thanks for the report on the oil variations,  I knew you guys would know.   8)
69 Z28 Burgandy

JKZ27

  • Hero Member
  • *****
  • Posts: 559
    • View Profile
Re: M21 Correct Oil
« Reply #7 on: July 21, 2015, 03:19:55 PM »
What is it about a synthetic GL-4 that is a problem? We use a BG Syncro Shift product that looks appealing but now I'm curious.
John
69 RS/SS Cortez Silver, L48 MC1
68 RS Ash/Ivy Gold 327EFI M20

bcmiller

  • CRG Member
  • *****
  • Posts: 4012
    • View Profile
Re: M21 Correct Oil
« Reply #8 on: July 21, 2015, 04:44:17 PM »
Synthetic just tends to be thinner and is more prone to leak. It's also more "slick" and you don't want that in a Muncie.  It was designed to use regular gear oil.
Bryon / 1968 Camaro SS 396 coupe - now old school 468 big block
1967 Camaro RS/SS 396 coupe L35/M40 - 4 generation family project
Looking for 68 Camaro with body # NOR 181016

rich69rs

  • Hero Member
  • *****
  • Posts: 1110
  • LF7/M35/Z22/Z87
    • View Profile
Re: M21 Correct Oil
« Reply #9 on: July 21, 2015, 11:41:30 PM »
Synthetic just tends to be thinner and is more prone to leak.

Although not transmission related, with both my '66 Chevelle as well as the '69 Camaro RS, after the engines were rebuilt in both cars I thought I'd do the right thing and use synthetics.  (The pros and cons of this has also been previously discussed on this forum.)  In my case, both cars would leak a few drops around the front engine seal - nothing serious, just anoying.  Went back to 10W30 and the problem went away. 

Our old rides are sometimes finicky when it comes to fluids.  Another example is using Dexron II ATF (originally Type A) in the power steering as per original specs and not the over the counter power steering fluid sold at the local parts house.
Richard Thomas
1969 RS

Mike S

  • Hero Member
  • *****
  • Posts: 2665
    • View Profile
Re: M21 Correct Oil
« Reply #10 on: July 22, 2015, 01:05:33 PM »
"In my case, both cars would leak a few drops around the front engine seal - nothing serious, just anoying.  Went back to 10W30 and the problem went away.  "

 I experienced exactly the same thing after my motor was rebuilt but with my 396 it was the front and rear seals that seeped slightly.
The dino oil put a stop to that.

Mike
67 04B LOS SS/RS L35 Hardtop - Original w/UOIT
67 05B NOR SS/RS L35 Convertible - Restored

L78 steve

  • Hero Member
  • *****
  • Posts: 797
    • View Profile
Re: M21 Correct Oil
« Reply #11 on: July 23, 2015, 12:45:55 AM »
GL-5 rating does not indicate a synthetic fluid. It can be either. Synthetic fluids tend to leak due to smaller molecules.
69 Z/28 Dover White. SOLD
67 SS/RS Mt. Green 1W,2LGSR,3SL,4K,5BY,07C. SOLD
70 Nova L78 Blk. Cherry,Sandalwood,M21,02B

bcmiller

  • CRG Member
  • *****
  • Posts: 4012
    • View Profile
Re: M21 Correct Oil
« Reply #12 on: July 23, 2015, 01:32:06 AM »
Nobody said GL5 meant synthetic. 

But nobody recommends using GL5 in a Muncie either.   Do it if you want, but it isn't good for the synchros. 
Bryon / 1968 Camaro SS 396 coupe - now old school 468 big block
1967 Camaro RS/SS 396 coupe L35/M40 - 4 generation family project
Looking for 68 Camaro with body # NOR 181016

Stingr69

  • Hero Member
  • *****
  • Posts: 901
    • View Profile
Re: M21 Correct Oil
« Reply #13 on: July 23, 2015, 01:10:47 PM »
Find GL-4 90W and be done with it.   GL-5 or synthetic is not an upgrade in this old transmission.

L78 steve

  • Hero Member
  • *****
  • Posts: 797
    • View Profile
Re: M21 Correct Oil
« Reply #14 on: July 23, 2015, 04:59:30 PM »
No one has stated the reason GL-5 is a problem in Muncie's. I will have to assume that it is too slippery for the braking effect the synchro's provide.
I have 3 Muncie transmissions with the GL-5 and I have not experienced any problems.
69 Z/28 Dover White. SOLD
67 SS/RS Mt. Green 1W,2LGSR,3SL,4K,5BY,07C. SOLD
70 Nova L78 Blk. Cherry,Sandalwood,M21,02B