600 weight oil is right. You can get it from most Model A vendors. Fill it until oil starts to come back out the fill "pipe" on the side of the ******.
And when you do drain and refill the engine crankcase put 4 quarts in then check the level on the dipstick...it is a 5 quart capacity but that is on a dry engine...about 1 quart is held in the dip tray.
And don't run your engine until the transmission has been re-filled....there are lots of moving parts in there that spin even if the car is in neutral and stationary....
I use 140w gear oil. But if you have 600w, use it......
Any good high pressure gear oil is satisfactory for spur cut gears and ball bearings if you want to get fancy use synthetic 85/140,it has superior properties to Dino oils due to consistency.