need suggestions on loosening a stuck fill plug on a fm 146 manual. tried wrench, corners were rounded. tried a pipe wrench, wrench, started slipping. ended up pulling the shifter and filling from the top. but would like to get the plug fixed somehow

 

If it is a square external (I have FM-146 and can't remember what I used) try pounding a 12 point metric wrench over it and then double up on the wrenches to get enough leverage to move it.