Just struck the same problem myself this morning. It's the first time I've changed the oil on this car, and the mechanic used by the previous owner was clearly a muppet and has managed to strip the sump plug drive while simultaneously torquing it up to about 6000Nm. The plug seems to have originally been a Ribe drive, but the idiot has probably been using a torx so he has completely wrecked it. I tried getting the correct bit in there and it seemed to be gripping OK for a start, but just cammed out as soon as I applied any serious torque to it. Tried a T50 torx as well, but has the same problem.
God knows how I'm going to get this off. Not much use having the right tools for the job if the last bloke didn't use them.
156 2.0 TS, silver with tart's boudoir leather, Eibach Pro-Kit and Koni FSDs