I use 10-40W motor oil in M41s, and F-type ATF in M46s.

Drain both the transmission and OD unit (J-types have 6 7/16"-head bolt to remove their oil pans). Clean the screens (there is two - one rectangular that sits in the oil pan, and a round one that sits in one of the three covers above the oil pan, the big one on the right side-you will need a special tool with two prongs to unscrew it)in the OD.

Filling it through the shifter is the easiest way to fill the "pudding-paddle" style shifter trannies. I have the car jacked up level, close up the transmission, remove the shifter, fill it up with fluid (~3.5 US pints)-> I lay down a towel just in case I spill a little oil inside the car, wait a couple of minutes, and then unscrew the fill hole plug to see if there is any overfill. Fill with more oil if needed.

Paul