Oil in the spark plug tubes can make it run rough, but not sure if that's your problem. Get the new rubber rings for the tubes and that should take care of the oil issue. As for the sooty plugs, that was likely due to the o2 sensor. When the ECU doesn't get a signal from the o2 sensor it will default to a "limp" mode. Since running too lean can cause damage, it goes to a rich mix. Running rich will create soot, rob power, waste gas, and eventually plug up the cat converter. Tim