Muncie 4 speed lube

[warren57]

I have read several posts on various sites that talked about what lube to use in a Muncie 4 speed. Most say use GL4 80/90 wt. They say never use synthetic or GL5.
The guy that rebuilt told me to use 100% synthetic Mobil 1. (local transmission shop that does only manual transmissions and gear boxes. Guy is about 70 years old and has been doing my stuff for 20 years or so, without an issue)
Does anyone have any thoughts? or reasoning I shouldn't do what my rebuild guy says?

[warren57]

After reading multiple articles it appears GL5 additive may eat up syncros...


GEAR AND SYNCHRONIZER WEAR PROTECTION
Most manufacturers of manual transmissions and
transaxles recommend an 80W or 90W GL-4 lubricant.
GL-5 gears oils which are required in hypoid differentials
are not used in most synchromesh transmissions
because the chemicals used to provide the extreme
pressure protection can be corrosive to synchronizers,
which are commonly made of brass or bronze. Typically,
the use of a GL-5 lubricant in a synchromesh
transmission will shorten the synchronizer life by one
half. The extreme pressure requirements of spur gears
and helical gears found in transmissions are not nearly
as great as found in rear-wheel drive differentials. A
GL-4 lubricant provides adequate protection for most
manual transmissions, unless a unique design
consideration requires the extra protection of a GL-5.

[chevynut]

That's interesting Warren. I always used 80/90 weight regular gear oil in my Muncie and T-10 transmissions.

I understand the Tremec 5 and 6-speed transmissions use ATF. Here's what they say...

"Due to various synchronizer materials used in TREMEC transmissions, and different climates and applications, no one single fluid will work for all transmissions. We recommend GM synchromesh part # 12345349, or Dexron III / Mercon Spec ATF fluid for all TKO transmissions. The transmission should be filled through the fill plug located on the passenger side of the transmission. Proper fill level is achieved when oil reaches the fill plughole when level (approximately 5.28 pints)"

[Rick_L]

Maybe I can clear at least part of this up.

The transmissions that have brass synchronizers require the GL-4 lube. They do not like the GL-5 because the additive package attacks the brass as said. You also don't want the 90W-140 weight as it is too viscous for good shifting.

Most of the later model transmssions like the Tremec have a paper lining on the synchronizer. This paper lining is very sensitive to the additive package and you need to use the recommended lube. ATF is a logical deal if the paper lining if they use a similar material to a GM automatic, which apparently Tremec does.

I don't know why you couldn't use a GL-4 80W-90 synthetic. I'm sure not all are GL-4, but I'll bet many brands are available in GL-4.

I've seen ATF used in Muncies before. I don't think it's the right thing because of the additive package and the low viscosity.

[warren57]

Filled it with NAPA Sta-Lube GL-4 today. That's worked for the past 50+ years and I'm not going to be around in 50 more years to see!!!