Sure looks like oil to me. The sludge on the plug reminds me of the days when I used "dino" oil before learning how to keep Volvo flame traps and engines running clean with synthetic.

Whether head gasket, valve guides, or a ring, I couldn't say, and don't believe compression or leakdown readings would have to suffer to prove it.

My approach would be to put in a fresh plug. See if the engine smooths out immediately. Then run a tank of gas through or feel the "missing" symptom again before pulling the plug to check how quickly it fouls again.

If I was wrong about the oil, and thought it might be flooding (have to use my nose) I'd consider swapping injectors or doing a volume test, but that's a lot of messing around.



--
Art Benstein near Baltimore

Two vultures board an airplane, each carrying two dead raccoons. The stewardess looks at them and says: "I'm sorry, gentlemen, only one carrion allowed per passenger."