1950 Chrysler Windsor semi-auto trans oil question

[Brooklyn Beer]

Changing the lubricants and doing an overall grease and lube job on my survivor 1950 Chrysler Windsor and see the trans calls for 10W motor oil in the shop manual. This is a low mileage mostly all original car but of course has a few weeps here and there.  Looks like the front and rear seal some.  Is there a brand of oil that is better then most for this application?  And would adding a seal conditioner screw the pooch?  I was lucky to have gotten the past owners (passed on) shop manual and with this section highlighted like it is (especially leaving no doubt about 10W only) it looks like he kept up with things but don't know when he stopped doing it. The rear end gear oil was a little low and I have noticed the shifting takes a little longer then it should so would like to change it out and see if that changes things up

[keithb7]

The transmission is a semi-auto correct? It shifts up when you life your foot off the gas? It shifts down automatically when you slow to a stop. 
 

Back in the 1950’s engine oil was not a multi-viscosity oil. When you changed your oil, you had chose the viscosity based on average ambient air temperatures. There was:

 

5W for use below as -10F

10W for use as low as -10F

20W for as low as +10F
30W for now lower than +32F

 

Today we have multi-viscosity detergent engine oils. For example a 10w30 flows like the old 10W at -10F. Yet it also holds up like the old 30W oil, at higher ambient temps. 
 

You can still buy straight 10W engine oil. Farm & Tractor type supply stores have it. I would not worry too much about what brand to buy. It’ll work. The manual calls for the oil to be changed every 10,000 miles. 

 

If your tranny is slow to shift today, it may have incorrect thicker weight oil in it now. As you drive, the oil pump in the transmission turns. It moves oil only when the car’s driveshaft is turning. That oil pressure, at 38 to 40 psi, waits for a series of events, to enable a shift.