The brief
The B200 had been leaving oil puddles where it parked, there was a burning oil smell with the engine running, the owner was topping up oil between changes, and there was visible oil and wetness on the engine. He brought it in. Those symptoms point at an oil leak from the top of the engine, and on these the valve cover gasket is the usual culprit. The valve cover seals the camshafts and the valvetrain in, and the gasket between it and the cylinder head hardens over years of heat until it stops sealing, so engine oil weeps out along the seam. That's the spots on the ground, the wetness on the engine, and the burning smell when it drips onto something hot. Left long enough, a low oil level can affect the engine's running, so it's worth tracing and fixing rather than topping up.
The diagnosis
On the lift the leak traced cleanly to the valve cover gasket, weeping along the seam, with the other seals in the area, the front cover, the sump, dry. The gasket had hardened past sealing, the typical failure on these. That's a gasket replacement, take the valve cover off and reseal it. The cover itself was fine, so it was a fresh gasket.
The work
The valve cover was removed, the old gasket taken off, and the mating faces cleaned back to bare metal. A new genuine Mercedes-spec valve cover gasket went on, the cover torqued back up to spec in the proper pattern, the oil that had tracked down cleaned off, and the engine topped to the correct level with the right spec oil. Then it was held at idle and checked to confirm the new seal was dry. A road test followed to confirm it stayed dry and the level held.
The outcome
Dry valve cover, no spots under the car, the oil level holding, no burning smell, and the engine bay clean. The B200 went home with the leak resolved. A valve cover gasket leak only spreads, and once it's letting the level drop the engine pays for it, so resealing it kept the job to a tidy one.