Where is the oil drain plug on a S500 Mercedes Benz?

I should be on the passenger side of the oil pan just a few mm from the bottom. The vehicle probably has a plastic cover that protects the entire underside of the engine, pull it off to access the pan. It should be a 13mm hex head bolt with a brass washer. The washer may be deformed a bit from it's last tightening, so either replace it, sand it smooth again with very fine paper, or carefully install it like it was before. If it's deformed and you reinstall it the wrong way it will leak. It should make sense when you see it... They hold a lot of oil, about twice that of a regular car, so be prepared with an extra large pan, or two pans. I use a 12qt rubbermaid storage container because the car will drop 8-1/2 to 9-1/2 qts. Btw, use good oil, Mobile 1 or better synthetic. The engine and it's maintenance timer are counting on it. It's also best to use a real fleece oil filter, not the cheezy crap that Fram etc sells. Buy it mail order or from the dealer.