The Stovebolt Site Forums The Shop Area General Truck Talk 1950 GMC Manual transmission fluid

Can someone tell me what fluid is used on my 1950 gmc standard transmission

90 wt gear lube

Farm Supply Store will have what you need.

Tractor supply has Ford Tractor Transmission Oil, a 90 wt mineral oil which is the factory specs. Not all stores have it in stock and is hard to search for on their web site.

Here is a link: https://www.tractorsupply.com/tsc/p...90-transmission-fluid-2-gal?cm_vc=-10005

I don't believe that there is a Tractor Supply Store in London Ontario or anywhere else in Canada.

Phak1 suggestion is what I just used in my 52 chevy 1/2 ton. Non foaming, noncorrosive to yellow metals and not too slippery for synchronizers. It also quietened it down some.

I use 75-90 synthetic.

What brand did you use and what is it's GL number?

Mike B

Thank you

Try Canadian Tire. They're about the same as Tractor Supply.

Mike, I use Wal-Mart full synthetic, GL5. Use this in the rear differential as well.

The older transmissions call for GL-1...GL-5 is for the rear and has EP (Extreme Pressure) additives which can attack the yellow metals in the trans (synchronizers), I don't know if synthetic attacks yellow or not.

One other issue with synthetic is it's very slippery and won't allow the gears time to slow down between shifts causing the gears to grind.

Mike B

GL-5 is for modern transmissions with small diameter, lightweight gears. Transmissions from the 1950's and before have big gears with a lot of weight and inertia, and running any kind of synthetic lube is absolutely the worst possible choice anyone could make. Add the extreme pressure additives used in rear axles with hypoid gears to the mix, and you've got a recipe for duisaster. "New" isn't always "better"!
Jerry

It was able to find some GL4 from Napa to use in my SM420 4 speed. I used to use modern GL5, but both the manual and posts around here say to use GL4.

So use GL4 in the transmission and GL5 in the rear?

Jerry, what should be used in the tranny and what in the rear diff?? You’re the professor! What say you?

The GL-4 / Gl-5 discussion seems to come up here every couple of months.

This paper goes into a lot of detail regarding various oil ratings and specifications. The author also provides a lot of good information regarding oil selection for vintage applications.

Transmission Oil Paper

He has written numerous other papers on engine oils as well.

Best Regards...

Just use what the factory used. No need for anything fancy.

Use our new search feature (the praying baby) in the left margin of every page. You’ll have hours of reading on the subject of gear lube, transmission oil and/or rear end oil.

Thank you!

This is what I found in the shop manual……

Transmission…… 6 pints of SAE 90. No mention of what type. I’ve read/heard mineral oil??

Differential……. 4 1/2 pints of SAE 90 “Multi purpose” gear lubricant.

I read Richard Widman’s paper on GL-4 vs GL-5 oils and haven’t seen in the manual where it mentions either for use in the tranny. Only saw SAE 90. (Screenshots below)

Please point me in the right direction.

Thanks

I'm not sure what shop manual you have, but my 1955 2nd calls for the transmission to have SAE 90 straight mineral oil...see attached picture.

This is what Tractor Supply sells as FORD Tractor Transmission Fluid.

https://www.tractorsupply.com/tsc/p...-all-mineral-90-transmission-fluid-2-gal

Mike B

As the author of the article mentions, a lot of confusion stems from the fact that the current rating system for gear oils was not around when lubricant for these older transmissions was originally specified. Trying to find a GL-4 or GL-5 reference in 1950's service information will be an exercise in futility, as those specifications did not yet exist.

If your service information specifies SAE 90 gear oil, then I would probably:

1) Just use, as Mike B and others mention, the 90 weight tractor supply stuff. It seems like a number of folks on this site use it without issue. (Although, it would be interesting to see the data sheet for this product to see how it compares with other gear oils)

or

2) Look at the viscosity chart in his paper, and note the range for SAE 90 gear oil. Surprisingly, it is pretty large. However. as he says, most 80W-90 gear oils tend to fall in toward the lower end of the range but should still work fine.

The main point, is to avoid using a GL-5 rated oil in an older vehicle with a synchronized transmission.

Best Regards...

just a quick note as ive been servicing my vehicles and changing all lubes in my trucks this week. In the transmission (sm420) I have used 90wt gear lube (gl4/gl5), 85-140 gear lube, and have even used lucas gear oil additive in the past. This time i switched to 90wt mineral oil from NAPA as suggested by many in this forum as well as original lube recommendations. After switching it was hard to believe the transmission was the same one as it shifted smooth as butter both up and down ... ill be changing lube in my other trucks as they come due for change

Experienced pretty much the same with my 52 1/2 ton just recently. Had an unknown combination of fluid in mine. When drained and refilled with 90 wt mineral oil it shifter smoother and quieter and quieter when running as well.