I put a brand new plug in last week coinciding with the oil change and decarbonization. I pulled the plug after all the problems to check the color and still looked brand new. In fact it looked scarey brand new, all shiney. Not a sign of a light tan colorization nor hint of oil residue. If I had to make a choice I would say the plug looked like a lean running engine being so clean and still factory white on the inside. Didn't 't think to try a different sensor. By the way, The plug is gapped at .022. Seems that's what I've always used for that engine. Is that what you're gap
ping your plugs at? Starting to think there might be an air leak somewhere. Might have to pull the motor and go through everything on a test bench.