Do not use syn oil for the primary. You would be throwing away money. Use whatever dyno oil is on sale- 5w30, 10w30,whatever.
ATF works fine and is helpful in determining if oil leaks are engine, primary or tranny.
The splash lube environment in the primary doesn't require a super special oil.
You're exactly right on mp. You can use practically anything in the primary. It doesn't have to be synthetic. Mine came with ATF in it and it was fine. Last time I changed it I had a spare bottle of Amsoil 20/50 so I just dumped that in.
Just make sure whatever you use is wet clutch compatible. Most of them are.