I had the same problem with mine this year (oil in spark plug 1 cavity).

I discovered a crack in the valve cover in the passage that lets cooling air into the covered center section. There is a web in the casting on the underside of the valve cover and the crack followed it. I will eventually get it welded or pick up another one but for now I cleaned it and put a bead of epoxy on the upper side over the crack.

I also degreased the circular spark plug opening seals and mating surface in the valve cover and added a thin bead of RTV for a better seal (not between the seal and head). Not enough that it squeezes out as you don't want that garbage floating around in your engine if it breaks off.

I was not able to positively tell if the oil was coming from the crack or the seal.

If your valve cover bolt seals are the original they should also be replaced (twelve 104 016 03 64) but probably not your problem in this case.

Edit: I used Permatex sensor safe RTV.