The dealer uses a suction pump to remove the oil from the engine. They do not touch the drain bolt. Chances are that the drain bolt has never been removed since it was installed at the factory unless you did a DIY oil change from the drain bolt yourself. Same thing happened on my C280.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record