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MT-1 Gear oil question

Westex

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El Paso, TX
I went over and 'prepped' my deuce I recovered in San Antonio two weeks ago; I added Valvoline GL-5 gear oil, which was rated MT-1. I just kind of topped off all of the drivetrain. Going back in this weekend to drive it home. Will be over 500 miles. Question is, as long as a gear oil is rated MT-1, is it safe to run in our deuces. In other words, does MT-1 rating make it bronze (yellow metal) safe?
 

OPCOM

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According to the oil company chemist I spoke to, yes, MT-1 rated GL-5 is not damaging to the yellow metal innards of the transmission or transfer. Mine's had 9000 miles on it with no noticeable wear or problems. It's made that way so they can sell a better lube for all the old equipment still in use today. Almost all GL-5 is now MT-1, and says so on the container label.
 

TDHofstetter

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I hate to dig up old bones... but I also hate to permit misinformation to lie hanging out there ready to snare newcomer after newcomer.

MT-1 gear oil is NOT suitable for synchronized manual transmissions, and it's NOT suitable for most (older, at least) transfer cases. GL-5/MT-1 gear oil is no safer to copper-alloy synchronizers or thrust washers than non-MT-1 GL-5 gear oil is; gearboxes containing any copper-alloy (copper, brass, bronze) parts REQUIRE a true GL-4 lubricant and should never be filled with any GL-5 oil.

GL-5 IS suitable for differentials, though, and is a better choice for them.

The problem is the molybdenum disulphide (moly-d) and other sulfur compounds used in the GL-5 gear oils; they're there to increase the oil's load-bearing and shear-resistance properties, but the sulfur attacks any copper alloy and so eats up our precious synchromesh rings and thrust washers.

Be warned.
Be wise.
Use real GL-4 in your transmissions.
 

glcaines

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Hiawassee, Georgia
TD is correct. Gl-5 should not be used with yellow-metal, which means it should never be used in the transmission or transfer case of a deuce. GL-5 has additives that are very corrosive toyellow metal. I use GL-1 in my transmission and transfer case as it has zero corrosive properties.
 

Coal Cracker

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Location
Weatherly, Pa.
I hate to dig up old bones... but I also hate to permit misinformation to lie hanging out there ready to snare newcomer after newcomer.

MT-1 gear oil is NOT suitable for synchronized manual transmissions, and it's NOT suitable for most (older, at least) transfer cases. GL-5/MT-1 gear oil is no safer to copper-alloy synchronizers or thrust washers than non-MT-1 GL-5 gear oil is; gearboxes containing any copper-alloy (copper, brass, bronze) parts REQUIRE a true GL-4 lubricant and should never be filled with any GL-5 oil.

GL-5 IS suitable for differentials, though, and is a better choice for them.

The problem is the molybdenum disulphide (moly-d) and other sulfur compounds used in the GL-5 gear oils; they're there to increase the oil's load-bearing and shear-resistance properties, but the sulfur attacks any copper alloy and so eats up our precious synchromesh rings and thrust washers.

Be warned.
Be wise.
Use real GL-4 in your transmissions.
I'm sure glad TD said it, so I wouldn't have to , I agree wholehartedly.
 
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sigo

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Leavenworth, KS
I read all the search returns on this subject awhile back and as I recall the MT-1/non-MT-1 debate was inconclusive. Strong arguments both ways. I wouldn't mind just erring on the side of caution and going for the straight GL5 gear oil, but I can't find any! I went ahead with the GL5 MT-1 for the diffs, but my trans and transfer haven't received new oil yet because I can't decide to go with the MT-1 or not.
 

Flyingvan911

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Location
Kansas City, MO
I read this yesterday, I think on another forum while searching on Yahoo. I can't say if it's correct or not but it is something to concider.

GL-5/MT-1 has sulfur in the EP additives. It takes a high temp (about 250 degrees) to "activate" them, then they corrode the brass. At low temps, the sulfur is "dormant" and doesn't cause and damage. The problem is that the teeth on the gears get hot then they side past each other. This little bit of contact and heat can activate the sulfur and it attacks the brass. So according to this, the MT-1 yellow metal safe properties go away with enough heat.

If this is wrong please let me know. I have GL-5 in right now but might switch to SAE 50. Are there other types of EP additives used in GL-5? Right now I'm running the basic NAPA 80W90 GL-5/MT-1 in the blue 5 gal bucket.
 
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Flyingvan911

Well-known member
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Location
Kansas City, MO
I think I'll try to get ahold of someone at NAPA and ask them if their GL-5/MT-1 has any sulfur in it. What are they using for their EP additive. If it does have sulfur, what would be the effect on a brass syncho with steel gears in our transmissions.

Someone has to know for sure. I might try several other GL-5 makers also.
 

TDHofstetter

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Bradford, VT
I actually got into a pretty good discussion with the boys at my own local NAPA when I went in to buy some GL4 last summer - they didn't realize that GL5 isn't an UPGRADE to GL4, it's just a different specification... and they didn't know that GL5 (with its large dose of moly-d) actually slowly destroys the copper-alloy parts in a synchronized transmission. They now know me pretty well. :) :) :)

As far as I know, moly-d is by far the bulk of the EP "additives" is GL5, whether GL5/MT1 or straight GL5. It makes sense for that to be true, since moly-d has fantastic lubricating properties on its own... similar to graphite. I use it in the machine shop for way lube on the milling machines & metal lathes.

FV911, you're right on the number about the temperature being extremely high right at the point of contact between two gears. All the power of the engine is focused at that point of contact, so there's fantastic pressure there - even if only for a moment per revolution. That pressure generates a lot of heat, leading to high temperatures. Heat is generated in the syncrhos themselve, too, because they're designed to work through brass-against-steel friction; they "scuff" tight" and "scuff" loose again every time you change gears.

Speaking of which... the addition of moly-d can also interfere with the proper mechanical action of those synchros even disregarding the fact that they're made of a copper alloy; gear oil with moly-d actually has too much lubricity for the synchros; the (correct) GL4 isn't as slick. That's why so many people who switch to GL5 (because GL4 is harder to find, or because they think GL5 is automatically better) wind up grinding gears from time to time - their synchros can't work properly.

My own final conclusion (whether or not it agrees with anyone else's) is that if your truck isn't much longer for this world and if you regularly subject it to really severe conditions (more than the Army would have subjected it to when it was new)... then use GL5 (MT1 or not) because of its ability to withstand vast abuse. Otherwise... especially if you (as I do) intend to drive your truck for a great many years... keep buying GL4. It's not very popular so it's a little harder to find, but it IS out there. Pennzoil still makes it (part #56076) and Amsoil still makes it. You can buy the CRC brand on Amazon (about eight bucks a quart). Don't even get me STARTED about those "color" (purple, red) gear oils...
 

Hottrodd789

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Orlando fl
I did some research valvoline 80w 90 gerl oil limited slip is gl4 rated and i called the tech line to make sure he stated yes in the valvoline the use an inactive sulfer aget wile other mfg.s and active sulfer ingredient!! i got 5.99 qt
 

paulfarber

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If you look at the lube order for M35a2 trucks they have been running GL5 since the mid 80's.

That means 20+ years of GL5 in the trucks and none of them have suffered from any soft of 'yellow metal failure'.
 

dodgedougak

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Joseph, OR
After talking with Chevron product engineers, I decided to use Delo synthetic transmission fluid 50 weight in the trans. No sulfer or EP products in it. I have more research to do before putting it in the transfer. I will stay with SAE 80-90 MT1, GL-5 in the differentials.
 

paulfarber

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Synthetic oils use a whole 'nutter chemistry to obtain GL-5 compatibility.

The cold, hard., ruthless fact remains:

Every M35a2 on the plant that has been in goverment service since the mid 80's has had GL-5 oil in every transmission/transfer/differential.

Absolutely nothing bad has happened.

If you spoke with the lube engineers, did they tell you that unless the operating temp of the Gl-5 oil/sulfur reaches 250F absolutely nothing will happen? And there has to be water? And that water boils off at 212F? Do the math.
 

RAYZER

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sanford/florida
Isn't gl 1 the proper lube to run in the deuce Trans and transfer case and not "gl 4”? napa carrys the gl 1 in gallon containers, gl1 is yellow metal safe.Don't be suprised though if the napa auto parts guy looks at you crosseyed when you ask for it. It's in their book.
 
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Clay James

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Location
Reno/NV
My trucks shift way easier with SAE 50 motor oil in the transmissions. A few times at stop lights I couldn't get it into first so I gave up and started in second. Doesn't happen anymore with SAE 50. Also put in 1qt of Lucas too, don't know if that made any difference.
 

doghead

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Synthetic in the trans, have proven to seize 2nd gear to the shaft.
 
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