My M20 took about a quart and a half. Buy 2 quarts and use what you need.
You want to fill it after it's installed so all the moving, tipping, etc. that's done during the install doesn't create a mess from leaks from the ends. In my case, filling was easiest to do through the shifter boot from inside the car- but be sure you do not drip any on the carpet! Use a big old towel, etc. to keep things clean- the smell lingers.
The correct fill level is similar to a diff- fill to the bottom of the plug- or slightly less (you just want to be able to wet a pinky finger when inserted).
Amsoil's 75w-90 GL4 MTG lube is what I've used. I had no experience w/synthetic lube like this before using it, but there have been no issues since using it and the tranny ran and shifted fine.
BTW, GL-5 spec lube is said to be hard on brass- don't use it unless it specifically says OK for use w/brass tranny components. Some synthetic GL-5 lube is said to be compatible w/the Muncie's guts- but check first! Or, just use GL-4 and don't worry about it. lol