Replace the plugs and monitor. It's my guess that this motor gets short trips and does not come up to full operating temperature, this results in sludge forming in all sorts of places, including the spark plug tips. The oil above the flats for the socket comes from the valve cover gasket, specifically the two pieces that have 3 circles, but the oil below the threads is coming from a source inside of the engine -- rings, valve guides/seals, those sorts of things. Another source would include things like the CCV system. I think you are having trouble with a vehicle that takes lots of very short trips in cold weather, in which case you can't do much beyond driving it more. I would put new plugs in and watch for the misfires to return. Perhaps in a month, pull the plugs again to inspect them.