Changed the oil in my C240 this weekend, it has close to 1000 trouble-free miles on it now.
You have to remove the rear belly pan to get to the oil drain plug. Its easy, with 6 screws holding it on. The front one can stay in place.
The drain plug is 13mm.
On the topside, the oil filter is very easy to change, and its a clean job compared to most cars! You simply unscrew the top, using the $7 factory tool, and pull out the filter assembly.
The filter assembly is odd looking, its a foot long, plastic rod with the filter pushed up all the way against the top. It took me a while to figure out how to get the filter off of the rod! At first, it looks like you have to disassemble it. But, you simply twist the filter cartridge part and gently pull it down. The new one snaps in place. There are also 4 O-rings that need to be replaced.
The manual says it takes 8.5 quarts, so that is what I filled it with. Mobil 1 15/50.
Buttoned everything back up, and all is well. Checked the computer oil level sensor, and it says "Level OK."
While I was under there, I took a look around. The front suspension is a multi-link that looks a lot like that on my 535i. The C exhaust is really cool looking, it looks like it all stainless steel, with large diameter pipes. It's cats are mounted way up high in the exhaust stream. It also looks like it has an additional 2 cats (for a total of 4??) further down in the exhaust, but those could be resonators, rather than cats. The cast iron manifold is the only part that doesn't look (from an appearance standpoint) really nice, or particularly efficient. It seems to me (from a totally non-scientific standpoint) that a set of quality, equal length tubular headers that "un-includes" the cats would be nice! I'm sure the cats are free-flowing, but they are mounted so high up, and near so many parts (suspension, engine, etc.) that its surprising that heat is not a problem.