I had the same problem on my '98 C230 this weekend, seized 13mm oil drain plug. I know it has never been off its whole life since it always had the oil changed at the dealer before. I had warmed the car up to operating temp before hand but then I considered that maybe it would be easier if it was not warmed up. Does anyone have experience loostening an oil drain plug cold vs hot? Anyway I had a good 6 point socket and lots of leverage but I didn't want to strip the head or shear it off so I though maybe if I could somehow cool the bolt it might shrink a bit (hey I'm a dreamer). Anyway, I had a can of "freeze-it" (spray for freezing things that electronics people use). So I sprayed the head of the bolt for awhile until it was cold but the pan was still plenty hot. I tried again and it came off. When I reinstalled it, besides using the proper torque (18 ft lbs) I put a little never-seize on both sides of the copper washer and on the threads near the head, just for "good luck".

Mike

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra