Oil can only get to the plug a few ways and the most common route is from worn rings (yours are new, so proper seating or application of the oil control ring might be an issue), and from valve stems. Some motors are put together without valve seals, and the clearance is tight enough on some motors that they don't need seals, such as the knurled bronze seals I used on a racing motor with no seals, intended to hold a bit of oil for lubrication purposes and not pass too much. I would check the valve stems on the cylinder you are seeing the trouble with to see if seals are in fact, even installed at all, and if so see if one is loose or broken. Sometimes a motor will sit at an angle and oil will pool up around a particular stem, and if the seal is not tight, oil will drip down the stem and contribute to blue smoke upon startup, but it will otherwise run perfectly. It looks like you are getting a lot of oil, so first thing I would go after is that valve stem (seal). Hope thats what it is, because that's the easiest thing to fix.
Oil can get into a combustion chamber from other means too, but for now I'd pop the valve cover and look at that offending cylinder from topsides.
Good luck! Keep us posted.
Paul
Here are some of the standard stuff below that you'll see on the net about how oil gets to spark plugs:
A spark plug shorted by excessive oil entering the combustion chamber is shown below. This is often caused by piston rings or cylinder walls that are badly worn. Oil may also be pulled into the chamber because of excessive clearance in the valve stem guides, or badly worn valve stem seals. If the PCV valve is plugged or inoperative, it can cause a buildup of crankcase pressure. This condition can force oil and oil vapors past the rings and valve guides into the combustion chamber
Worn valve guide seals and guides can allow oil to be sucked down the guides and into the combustion chamber. A heavy buildup of thick black deposits on the plug and intake valve would indicate such a problem. Worn or broken rings, or damage to the cylinder wall can also allow oil to enter the combustion chamber and form ash deposits on the plugs.
Oil Fouled Plug
Has wet-looking, black deposits indicating bad rings or valve seals