I drive a 1980 245 M46 also. I can't tell you the thread (I think it's like a pipe thread) but what you need is the proper tool. You can purchase an 8 point socket and have no need to change anything. A 7/16th" size socket should work just fine. If there's such a thing as an 11mm 8 point socket that would work too. OR -- back in the old days when square shaped drives were common (tranny and diff plugs, brake shoe adjusters, etc) you could buy a wrench with boxed square ends.
I've always used a normal Type F auto trans fluid in mine (310k) but I would think a compatible synthetic would be ok -- unless -- the synthetic is not compatible with the OD conical clutch. Maybe someone with experience with that aspect will post. Also -- I think you meant to type "M45" - not 47 in your last sentence. -- Dave