O Ring, Rocker Cover
O rings are inexpensive by design. Take the long view and change these every time you inspect or adjust the valves. Under the high closure force generated by the Rocker Cover's wire loop, these round section O rings will eventually take on a square section, at which point Oil seepage may start to occur.
With all that air flowing backward, the next stop for escaped oil is your exhaust system, where black caked oil directly in line with the suspect O ring is a great clue.