1939 Plymouth Trans w/OD WHAT FLUID

[Sactownog]

I am still clueless on what fluid to use for my transmission which is a 39 Plymouth 3 speed with OD. There are two fill locations. 3 speed front area and rear OD area. 

 

I have talked to amsoil and didn't really get an answer. 

 

we settled on Gear oil that would be 75W-80 or something thin like that. but I got no definitive answer. 

 

anyone got the true best trans oil to use? 

[keithb7]

So I understand, you have a manual 3 speed up front? It has a synchromesh engaging 2nd and 3rd. Do not use a common modern GL-5 in there.  Use Red line MTL.  Or you can use a GL-1 that is available at your local Napa in the USA. (GL-1 is not available at Canada Napa locations) The overdrive I'm not 100% sure. I've never owned one.

 

Modern GL-4 or GL-5 is too slippery. The synchro speed rings won't speed up or slow down properly to match speeds. You'll hear grinding as you shift.

 

https://www.redlineoil.com/mtl-75w80-gl-4-gear-oil

 

https://www.napaonline.com/en/p/FNJ65201

Edited June 24, 2020 by keithb7 (see edit history)

[Sactownog]

ANYONE ELSE: got some good advise. I agree with @Keithb7 about the GL1 fluid. just wondering if anyone else has advise. I am leaing towards the NAPA GL1 gear oil. please respond asap. 

[Grimy]

My 1948 Jeepster (with B-W overdrive) owner's manual is explicit that only "straight mineral oil" (i.e., GL-1) be used in the transmission and overdrive (separate fill plugs), but that Extreme Pressure (EP) gear oil (i.e., GL-4) be used in the differential--and cautions to not mix them up -- but not in those exact words.  I agree with @keithb7 that GL-4 is too slippery for the synchronizers of the day.

[keithb7]

Just for the record, I am currently running Caterpillar TDTO 30W. (Transmission Drive Train Oil). Working very well in my 1938 3 speed manual. 
 

Available at your nearest Cat dealer in 1 gallon jugs. 

[Brooklyn Beer]

NAPA 90W SAE    GL1.  Just used it in my 39 Chrysler with OD.  Tossed in some seal conditioner with it.  remember that it takes some time for the fluid to run over and fill the rear so a slow fill is needed.  I just pumped up the fill hole and shoved a rag around the hole and overfilled it.  Let it sit for 8 hours and removed the rag and let the excess drain out.  They come in gallon jugs.  Are you saying you have a drain and fill plug for both?  4 plugs total ?

Edited July 5, 2020 by Brooklyn Beer (see edit history)

[Sactownog]

On 7/5/2020 at 9:24 AM, Brooklyn Beer said:

NAPA 90W SAE    GL1.  Just used it in my 39 Chrysler with OD.  Tossed in some seal conditioner with it.  remember that it takes some time for the fluid to run over and fill the rear so a slow fill is needed.  I just pumped up the fill hole and shoved a rag around the hole and overfilled it.  Let it sit for 8 hours and removed the rag and let the excess drain out.  They come in gallon jugs.  Are you saying you have a drain and fill plug for both?  4 plugs total ?

YES! i have two fill holes and 2 drain plugs for fluid. one on tranny 3 speed area and one on OD unit. 

 

another quick question. my trans has a push pull rod for the OD. anyone know on the trans if the tab is pulled forward (towards engine) is that disengaged or engaged in to OD 

[Brooklyn Beer]

all the way in is OD and freewheel.  Manual says at any speed above 25 mph if you take your foot off the gas pedal for a brief couple seconds it will drop into OD.

Edited July 7, 2020 by Brooklyn Beer (see edit history)

[Larry Schramm]

You can also buy GL1 fluid at tractor supply.