M35A2 Transmission Fluid Change

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Dasgog

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Hello!

Alright so I have looked at several threads in regards to what fluid to use when changing my Transmission fluid and really all it did was confuse me more. Mainly because of the vast opinions of what I should use! I understand there are a lot of factors and also a lot of personal perspective .
I am here in AZ and I believe things run hotter...

As far as the procedure goes I got that, no concern, but I am on the fence with what to really use. It’s a 1972.

thank you for any help and really for all the help I have always received.
 

doghead

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As far as i am aware of, there are no new developments in oil technology that should change how you decide to use the data and opinions posted in this forum.

Gear oil works fine, has for over 50 years. Probably will for another 50 years.
 

Menaces Nemesis

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I feel your pain... Seems there's been quite a few lube opinion battles here on SS. Some folks swear by the non-detergent straight-weight motor oil, my trans hated it, especially when hot, made it a real PITA to shift. Runs much better with the gear oil in the link below, and it's nice to have just one lube for the trans, t-case, and diffs. Fine print on this gear lube says it's good for all GL-2, 3, 4 and 5 applications, and a cheap lube pump from harbor freight makes the job so much easier...

https://www.oreillyauto.com/detail/...-gallon/80041/4495694?q=masterpro+80w90&pos=2

https://www.harborfreight.com/gear-oil-and-fluid-pump-61746.html?_br_psugg_q=gear+lube+pump
 

cattlerepairman

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I ran 80/90 gear oil in my transmission. That worked fine and I agree with other posters that in a warmer climate it is probably all you need. Where I am, even the margins of the winter (spring and fall) are cold enough to make initial shifting a very stiff affair.

After my OD mod I now run 40w straight non detergent oil. Shifting is also fine but the oil is thin. I did not overfill but on the first longer highway run the tranny puked quite a mess out from the front. Seems fine now and I leave it alone. I am considering switching to 50w.
 

NY Tom

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This thread could go on for a long time based on others...
I tried 50W Mobil Synthetic made for manual transmissions and my 1970 M35A2 did not like it. Tough to shift.
Switched to an ISO150 (~40W) straight non-detergent gear oil type product we use at work for stamping press recirculating lube and it seemed to be happier.
When I got the truck it was filled with 85W90 and shifting when cold was really tough.
The 40W seems good but leakage has increased unfortunately.
Transfer case was the same problem - filled with 85W140 and very hard to shift from high to low when cold. Switched out to 85W90 - shifts much better and leaks way more too...
 

frank8003

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The ancient question (circa 1960) of to use detergeant oil or non-detergenat oil answer is to see if that particualr lube system is circulating and pressurized.
If it is a gearbox of normal machines and is circulated by action of the parts but not by pump then the basic design is to allow that which does not belong in the lube oil to settle out to drain plug area. When lube liquid is changed out then the nasties go with it as per design.
As per the actual viscosity itself of lubes there is a chart. This does not have to do with additives.
Viscosity Chart.jpg
 

Menaces Nemesis

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From a post I wrote in a similar thread a few years ago...
FWIW, I believe transmissions could possibly have different druthers... The same way one brand-new engine may have more or less blow-by than the next, what works well in one trans may not work as well in another. One trans may have a bit tighter or looser clearances in a spot or two, maybe the synchro detents and springs are a bit grabbier, or more forgiving, in one trans than another. With these variances, maybe one trans prefers something that's a bit more viscous, or maybe less, depending on temperature and how it's being used. I've tried the straight-weight Non-Detergent oil, and the synthetics too. When using 50wND in my 3053A, it was flat-out noisy. After getting off the highway from an extended run, shifts from 1-2 with both the 50wND and Synthetic 50w were really reluctant and sticky. Mine seems to run quieter, cooler, and shift best with 80w90 GL4/5. YMMV.

I do drive my truck year-round, but only when temps are above freezing. I figure after 50+ years of service, the ol' boy is retired, the engine, all the seals and gaskets and other "soft parts" deserve to sleep in until it's above freezing.
 
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Menaces Nemesis

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Another consideration is availability... if we're just looking for straight-weight oil without the "non-detergent" characteristic, that's easy. So's non-detergent oil in lighter viscosities (compressor oils). Seems the manufacturers of heavier weight non-detergents have kinda been phasing out the 40-50 wt smaller containers/consumer market (probably not much demand?). Although 40 and 50 wt non-detergent might still be had if one knows where to look, often times it won't be on the shelf of most local auto parts stores.
 
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Katavic918

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I have always used 90w mineral gear oil. Napa has 5 gal buckets for I think $60. Good luck with your decision. I don't see an end in sight for this thread.
 

fleetmech

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The lack of consensus has left me a little reluctant to change the trans oil in mine, but I eventually picked up some ND 30wt from tractor supply. Its on the list to be changed within the next few weeks. My '70 shifts ok as a rule, but a bit slow when cold especially that 1-2 shift. Mine has gear oil in it, but I'm switching to see how it likes the change.

FWIW I have (and put a lot of miles on) a 1969 Diamond Reo gasser (civilian truck) with the non-od variant of this same transmission that I filled with regular old 80w-90 long before I knew how the 'net worked. I remember being a bit surprised that the 1-2 shift seemed to actually get a bit stiffer with the new oil, but I wrote that off as being just a quirk. Bearing in mind that the previous owner ran it out of trans oil until the pto seized and stopped the truck cold, then sold it to me with about a quart and a half in it (first and only time to date ive felt a gear stick get that hot) I think most any lube in these things is going to be ok. That was close to 10 years ago and while that truck is now retired waiting on major chassis repairs and re-purposing, last it ran a year or so ago, the trans was still fine.

I find it interesting that this question also crops up a lot in the world of 'regular' classic cars as well, and I'm not aware that there is any real solution there either. With the knowledge I have now about the so called 'yellow metals' I would probably avoid any bottle that says "for un-synchronized transmissions", but my old Reo appears to be proof that it either doesn't matter or, as Menacers said above, is dependent on the whim of your particular gear box.
 
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