After obtaining an oil pressure tester I don't believe my problem is the cam plug. My oil pressure is consistently between 60 and 70 psi. Cold, warm, hot, low rpm, high rpm. My next thought is the oil pressure sensor is going wonky, so I've ordered a new one. At least I can cut grass in the time being (it's getting long!).
While I was looking things over I noticed my fan belt was loose and made a wire retention jig to keep some wires off of the exhaust pipe. I also wanted to adjust my clutch, but i'm not sure if i got it right. I think maybe my clutch is worn? I seem to remember there being more pedal when I got the tractor, but now it seems to be pretty grabby. I have a kubota workshop manual, which has a section for adjusting the clutch, but the book is pretty sparse on details. It simply says there should be about an inch of play in the pedal, but at what point in the clutch travel does one start to consider it "play"? If I move the clutch arm two spins back then the safety switch won't let the tractor start, but two spins forward and clutch doesn't fully disengage with the pedal and the starter grinds. I put it back where it was, but makes me think my clutch is wearing down.
Another question, is it normal for the breather tube coming off the top of the engine to be puffing? No smoke, just puffing lots of gasses.