Thanks for the advice!  I've seen that GL-4 140W at my local NAPA, so I know its readily available when I get time to change out those fluids again.  I've been working on my technique with some success.  In particular I now have the double-clutch downshift into second just about mastered.  I don't have a tach handy so I don't know my exact idle speed.  My wild guess would be maybe 600, so I could try it a bit slower.  FYI: it runs pretty close to 6MPH idling in first gear which I had read somewhere was the right idle.  I also see that you are shifting sooner than I usually do.  I've been shifting out of first at about 10MPH and out of second at a little over 20MPH, although lately I've had better luck shifting out if first "as soon as its rolling" (so maybe 5MPH?).  I'll try shifting at lower speeds.  I've had lots of experience (when I was younger) with balky three speed trannies so I know I'll be able to bypass most of the grinding with practice.  I was just wondering if a different lube would overall be "better" for the system :)

My experience with the 1933 echos that of 32cabrio: I run a 140 wt hypoid gear lube. I find that it is harder to get a silent shift with 90 wt oil. Under no conditions do you want to use the old 600 wt. "steam cylinder" oil in your car!

Your gears may be constant mesh but they are engaged by sliding "dog clutch" (as I recall the term from my manuals). Unless the dog clutch is spinning at the same speed as the gear it is trying to engage you will get some noise. You can match speeds (synchronize) the dog clutch and the gear by "double clutching". For upshifting it is easiest to simply be gentle and upshift at a pretty low speed. For downshifting, engaging the clutch while in neutral and blipping the throttle will speed up the dog clutch. If you do it right it will be spinning at the correct speed to engage the lower gear.

The difference between this transmission and a more modern one is that the more modern one (1935 and up on Plymouth) has a lock out to keep the teeth on the dog clutch from touching the teeth on the gear until they are spinning the same speed. And they also have a spring loaded friction clutch built into the setup to get the dog clutch speed to match the gear. The combination of the friction clutch and lock out are called a synchronizer or synchro and a transmission so fitted is called a synchromesh transmission.

At least you are not driving a 1920s era car where the actual gears slide to engage. That requires noisy square cut gears and even more skill to make a silent shift.

The modern lubricants for the tranny and rear end are : 90 SAE gear oil for the tranny? and regular hypoid fluid for the rear end. I use Pennzoil products in both cases. Your problem is the tranny in a '31 PA is a 'crash box', no syncromesh. And square cut gears in everything but 3rd. The best way to keep the noise down to a bearable level while shifting is to 'double clutch'. That is; when the gear shift lever passes through the neutral gate, pause and let out the clutch. This will allow all the gears to more nearly assume the same speed and then put the clutch down again and complete the shift. With practice this will become automatic and really doesn't slow the shifting down. But I would practice away from traffic til you get the hang of it. The reason the box is crunching is because the gear sets are moving at different speeds. Syncromesh keeps the speed of the gears the same and the teeth mesh in the valleys instead of hitting on the tops of the opposing teeth. Hence the crashing sound. The gears were also eventually cut with a bevel and this quietened the running noise down and also made for easier shifts.? I don't know what year syncromesh came out but it was many years after '31.? ?:o

The 31 trans is close if not exactly like the '32 which I have. I use GL-4 140.  It is heavy enough to slow the gearset down as you shift. NAPA has a straight 140wt. API/GL4 made by Sta-Lube which is a mild EP and will not harm yellow metal; bronze, brass, etc. I use it in the '32 which has an O/D as well as other cars that I have. No problems and it doesn't leak as easily as 90wt. Also, driving technique has alot to do with gear clash. I try to be thru the gears and into 3rd by 15 to 18 mph max. I also found that getting my idle speed down to 450rpm or so allows the gear speed to be slower and helps to avoid clash. I hope this helps. 

I recently had the transmission and differential lube replaced in my 31 PA.  The heaviest oil the shop had was 75W-90.  Since then, it seems that the shifts (especially between second and third gear) are now much more prone to "grinding".  I've read that this model should be "constant mesh" between these gears so grinding is puzzling.  I've also read that some of the older cars need a very thick gear oil (what was called 600W "steam engine" oil, but now has different designations).

SO: I'm hoping that the "early thirties" plymouth owners can chime in on what gear oil works best for them and whether changing to something thicker is likely to ease the shifting.

...thanks in advance :)