As Klaus mentioned. use a 6 point socket. The drain plug in aluminum engines always is hard to remove. Honda 450s were terrible, you needed a ratchet, a six point socket, ans a LONG box wrench to aid in pulling the ratchet.

ALWAYS use a new aluminum washer each time you put the plug back and be sure to use a torque wrench to assure that the plug is tight enough, but not too tight.

The plug is probably tight because the last guy did not use a new washer and had to tighten the old washer too much.
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '95 855, '95 854, the first three are mine, heh, heh, 435,000 miles put on 9 bricks