As Joe said, the trans requires the use of lubricant with reduced friction modifiers. The MX4T is used in bikes with common crankcase/clutch (wet)/transmission sumps.
Your trans in a 2 wheeler is NOT a modified auto transmission, but a specifically designed transmission for the Boss. Do not use ATF......
There has been a lot of talk about the Amzoil, but I myself would not go with it unless NESCO blessed it. A call to Jim @ NESCO might answer your questions. The MX4T is 10W-40.
I just changed mine after 4000 miles on the #7 converter and found it was almost clean enough to reuse (I didn't) and no appreciable amount of debris in the filter or on the pan. I gotta believe the fluid is the right one for the tranny, 'cause I'm not particularily easy on mine.