Last summer, my flat-land based ‘09 S550, while in the mountains of Colorado gave me a burning oil smell a few times. I think it was after quite a bit of hard engine braking. While in Breckenridge for a few days, I inspected the engine and my nose took me to the passenger side and of course I was seeking something dripping on the exhaust so my hand went to the rear of the engine. Reaching under the head, my fingers had a very small amount of oil on them. I have never smelled burning oil again, so I let it go. On another thread, I was discussing changing the “engine mounts” (thread title), and while I had the bottom pan covers off, I noticed a stain on a rearward one below the same area where I felt the moisture under the head.
I looked up and saw oil weep stain. Sorry no photo, but to me, it looks like a breather attachment with three or four bolts screwing into the back of the head. There is a tube going across back of engine attached to the piece bolted to the head.

What parts to order? Easy enough to change?