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The best lube for a T10??

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ramblinrev View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ramblinrev Quote  Post ReplyReply Direct Link To This Post Posted: May/20/2019 at 10:09pm
Thanks, everyone.
I put Pennzoil Synchromesh in for now.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FSJunkie Quote  Post ReplyReply Direct Link To This Post Posted: May/21/2019 at 2:05am
The manuals specified SAE 80 gear oil, SAE 20W-20 engine oil, or SAE 10W-30 engine oil.

Synchromesh transmission fluid is meant for transmissions that specify automatic transmission fluid. Many later manual transmissions specify ATF. Synchromesh fluid is not heavy enough for these old gear boxes.

My T10 did not shift well on synchromesh fluid and leaked like all heck.

NAPA sells GL-4 80W-90 gear oil made by Sta-Lube. I use that.

They also have Quicksilver marine SAE 90 GL-4 for outboard units. That would probably work well too.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steve_P Quote  Post ReplyReply Direct Link To This Post Posted: May/21/2019 at 9:49am
Originally posted by FSJunkie FSJunkie wrote:

The manuals specified SAE 80 gear oil, SAE 20W-20 engine oil, or SAE 10W-30 engine oil.

Synchromesh transmission fluid is meant for transmissions that specify automatic transmission fluid. Many later manual transmissions specify ATF. Synchromesh fluid is not heavy enough for these old gear boxes.

My T10 did not shift well on synchromesh fluid and leaked like all heck.

NAPA sells GL-4 80W-90 gear oil made by Sta-Lube. I use that.

They also have Quicksilver marine SAE 90 GL-4 for outboard units. That would probably work well too.


I've never seen engine oil recommended for a T-10. What's your source for that? Gear oils have specific additives that engine oils don't.

I've not used the Pennzoil synchromesh so I'm not familiar with it. But it doesn't list a viscosity so that makes me think it's more suited to some modern manual transmissions as said above. My 2009 Tacoma specifies 75W-90 GL-4 or GL-5 for the 5 speed manual trans.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: May/21/2019 at 11:48am
Synchro is not meant for transmissions that call for ATF.... and no, don't use engine oil. Especially in the T-10
That was spec'd for a while in some of the T5s but I'd never use it in an older box. 

Double-check what Steve said - the bottles I have mention GM and Chrysler specs and part numbers. 
G5 is only for transmissions without the yellow metal so yes, some will indeed specify that - but not for our older transmissions and not for T5.
Synchromesh matches GM part number 12345349 spec 9985648 or GM part12345577
and Chrysler part number 4874464 spec MS9224

I think I have those numbers right.....

Steve is correct in that gear lubes for manual transmissions are formulated very differently from motor oil. 
Even AMC changed their mind in the 80s - and dropped the motor oil recommendation.
Do keep in mind that hypoid gear lube is also different so don't use the same stuff you stick in your differential. Different needs, different formulations and hypiod gear lube is typically lead to harder shifting.
AMC also dropped the ATF recommendation - quickly, I might add, based on the TSB and TSM supplements I have. 
We stopped using ATF in sticks years ago (when I worked on Jeep transmissions, etc.) - had nothing but trouble with it.
What the TSM says isn't always the bottom line or last word - often the specs were changed after the TSM came out so be very careful - and Chilton and others followed the original TSM release - I can show that - Chilton and Mitchell have specs listed for some AMCs that AMC changed - but the other manual publishers never bothered to re-check, so they are wrong. Examples include alignment specs AND transmission fluid recommendations! So be careful what you see in a manual - check bulletins and subsequent releases for changes. AMC had tons of troubles with transmissions and changed their specs multiple times for SOME transmissions. Note I said SOME........

Marine fluids are also intended for different applications - higher speeds, but not the harsher environment of a synchronizer.  It could be an expensive experiment to assume...... you have different gear cuts in those lower ends. 

By the way - if a transmission leaked it had bad seals - the fluid didn't cause it. They all need to be sealed well and all use the same types of seals so if a transmission leaked in synchromesh, it's got bad seals, period. It's just that a thick lube is masking those bad seals.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote purple72Gremlin Quote  Post ReplyReply Direct Link To This Post Posted: May/21/2019 at 12:43pm
Originally posted by billd billd wrote:

Synchro is not meant for transmissions that call for ATF.... and no, don't use engine oil. Especially in the T-10
That was spec'd for a while in some of the T5s but I'd never use it in an older box. 

Double-check what Steve said - the bottles I have mention GM and Chrysler specs and part numbers. 
G5 is only for transmissions without the yellow metal so yes, some will indeed specify that - but not for our older transmissions and not for T5.
Synchromesh matches GM part number 12345349 spec 9985648 or GM part12345577
and Chrysler part number 4874464 spec MS9224

I think I have those numbers right.....

Steve is correct in that gear lubes for manual transmissions are formulated very differently from motor oil. 
Even AMC changed their mind in the 80s - and dropped the motor oil recommendation.
Do keep in mind that hypoid gear lube is also different so don't use the same stuff you stick in your differential. Different needs, different formulations and hypiod gear lube is typically lead to harder shifting.
AMC also dropped the ATF recommendation - quickly, I might add, based on the TSB and TSM supplements I have. 
We stopped using ATF in sticks years ago (when I worked on Jeep transmissions, etc.) - had nothing but trouble with it.


By the way - if a transmission leaked it had bad seals - the fluid didn't cause it. They all need to be sealed well and all use the same types of seals so if a transmission leaked in synchromesh, it's got bad seals, period. It's just that a thick lube is masking those bad seals.


If Im wrong on using synchro mesh fluid in a T10, Id rather know now so I dont make that mistake. 

And a manual transmission should not leak period.....

About ATF in a standard....my father bought a 1964 Dodge Dart new, it had the 3 on the tree....shifted terrible.....was still under warranty.  Dealer drained the ATF.  Put 90wt in. Shifted much better.  Dealer charged him for it and said the Chrysler was doing ATF as an experiment....and Dad wasnt happy. He had problems with the car... so it was the only Chrysler product he ever owned.....
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bandana Quote  Post ReplyReply Direct Link To This Post Posted: May/23/2019 at 6:12pm
summit carries the 80w-90 GL-4 PennGrade at $9/at. Got a few quarts on the way to try in mine. Been using a synchromesh this past year or two.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote nda racer Quote  Post ReplyReply Direct Link To This Post Posted: May/24/2019 at 1:00am
Originally posted by Steve_P Steve_P wrote:

Redline makes GL4 gear oil. It's not cheap, and you probably won't find it locally, but it's on Amazon...

Summit also. They have a couple of different GL-4s but this weight is what I run in a Saginaw right now. It came extremely highly rated and made a big difference for us.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote FSJunkie Quote  Post ReplyReply Direct Link To This Post Posted: May/24/2019 at 2:55am
Both my 1965 and 1966 manuals say motor oil is allowable in the 3 speed, 4 speed, and overdrive transmissions. Argue for or against it all you want, the point is AMC allowed it regardless of what we think and I relayed that information as it was written by the writers at American Motors.

Some people have to live with transmissions that leak. For those people, a thicker (but still approved!) fluid that doesn't piss out the ancient dried up seals is a big plus.

I use Sta-Lube GL-4. It shifts better with it than on synchromesh fluid, it runs quieter than on synchromesh fluid, and it doesn't piss out the seals faster than I can refill it.

Edited by FSJunkie - May/24/2019 at 3:09am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: May/26/2019 at 8:00pm
i ran 10W30 or something, 80W90 or whatever it is, and 20W-50 in my T96. all were fine. i doubt it matters at all, unless you're in Alaska or Death Valley (viscosity).

none of them helped the T96 from being a weak piece of crap however.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: May/26/2019 at 8:48pm
Originally posted by FSJunkie FSJunkie wrote:

Both my 1965 and 1966 manuals say motor oil is allowable in the 3 speed, 4 speed, and overdrive transmissions. Argue for or against it all you want, the point is AMC allowed it regardless of what we think and I relayed that information as it was written by the writers at American Motors.

Some people have to live with transmissions that leak. For those people, a thicker (but still approved!) fluid that doesn't piss out the ancient dried up seals is a big plus.

I use Sta-Lube GL-4. It shifts better with it than on synchromesh fluid, it runs quieter than on synchromesh fluid, and it doesn't piss out the seals faster than I can refill it.


Allowable - but did you look at the TSBs or TSMs or TSM supplements later?
You'll find AMC changed their mind more than once. Owners manual is a guide - but as I can prove, a guide only, as in at least two cases AMC said NO NO NO - don't do that. In fact I have two TSBs issued the same year that changed transmission lube. It started with one lube in the owner manual, then it changed, then a TSB changed it and then about a month later another TSB changed it. 
The owner manual is a starting point - sometimes it sticks and isn't changed, sometimes it DOES change. I guess I could spend time going through all of my 1960s and 1970s TSBs to see....

By the way - I ripped up three 3 speed transmissions in my first car inside of two years.
I kept ripping up the synchronizers.
We also changed synchronizers in a lot of AMC transmissions from 1960s cars - and we stopped using "motor oil" because as the boss said "that's for motors". (he ran the shop for Charter AMC dealership in the 50s and 60s and in the 70s I started working there.)

I guess that's why I have some of the Kent Moore transmission tools from that era - we worked on a lot of manual transmissions. I guess that says something, eh? Real life experience in a shop back then - dozens of transmissions. We never put engine oil back in once we worked on them. 

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