LOL.... u r finding all the fun stuff.....
My thoughts are that those drain plugs get so wickedly tight from the sand castings.... where as the porous sand cast will actually suck up any lube on the seat.... its crazy..... that's for sure...
I had one several years ago... I had to put a big ol pipe wrench on it to bust it loose, tapped the wrench with a hammer. I know the feeling of maybe going too far.... but face the music, if you don't get it off, its not going to be any worse than cracking that case (very doubtful)....
I don't know if you have NAPA over there or something similar, but notice on that cross over reference parts numbers... there are nutted drain plugs out there that work better.... I have the NAPA drain plugs, and they don't seem as tough to get off....