what oil to use in 1950 Merc 3speed?

I got some amsoil from rodncustom supply for the rearend but I'm trying to check the oil in the actual trans at the same time. What should I put in the trans and how much of it. Also where to you drain and fill these 3speeds?

Thanks everyone,
Edgar

 

There should be a drain plug on the bottom of the trans case. Use 90 wt gear lube in the trans. You have to put it in through the plug on the side. Just fill it until it starts to run out of the filler hole. If it's an overdrive trans fill the rear portion also.

 

If there isn't a drain plug I have sucked out the oil with a syringe gun and surgical tubing before.