Yes, you were right. The drain plug is rusty like **** and get rounded. I tried to un-screw it and it was totally rounded.How so? Did the female allen hex round out? Mine was quite rusty and rust-rounded, so I'd welded a nut to it to remove it. Use a GM drain plug for an of their 6 speed transmissions (get one at the auto parts store) and cut the smooth, non-threaded end off it (that is beyond the threads) and use that. It has an honest 14mm bolt head for easy removals in the future, and an integral rubber o-ring for sealing. An 8 mm allen is too small to apply much torque when they corrode.
Wait a minute, I used a star bit, maybe I should try hex bit. It is cold here now, wait till the weather is warmer. Thank.