I can't believe that you're leaking fluid from a spark plug hole. Like Paulster2 above, I'd like to see what happens when that area of the motor is cleaned thoroughly so we can start looking at this thing fresh.
On the opposite end of the spark plug is a cylinder full of fire and very high pressure. If anything leaks into that cylinder (say during idle time or sporadically during intake strokes), it'll be introduced to that fire shortly. If it's flammable, it'll burn. Oil's flammable, which is why the upper end of the combustion chamber is so hard to keep lubricated - the stunningly thin layer of oil that gets deposited there on every piston upstroke gets burned away during the power stroke. LARGE amounts of oil - too much to burn - wind up very quickly fouling the spark plug, resulting in a misfire.
If liquid leaking into the combustion chamber is NOT flammable, it gets vaporized by the heat and pressure of combusion on every power stroke. The only qualifying liquid would be water-coolant mixture, and we know what leakage of water-coolant into the combustion chamber looks like - clouds of dense white smoke.
If ENOUGH nonflammable liquid reaches the combustion chamber to not be fully vaporized, again we wind up with a misfire, and we should expect that leakage to continue after the motor is shut off because the engine's still hot. Such a continued leakage into an idle cylinder would likely build up quite a charge of it in there, too much to compress on the next compression stroke. Result: hydrostatic lock, itself definitely leading to a no-start condition and possibly leading to catastrophic damage to the piston.
The seal between a spark plug and the head isn't a marvelous seal, but it's really pretty good. Always. It has to be, because it has to hold up against potentially thousands of pounds of pressure during every power stroke with no detectable leakage - if it leaked exhaust gases during power strokes, you'd be able to feel hot puffs of those exhaust gases around the back of the plug. If it carried liquid, it wouldn't seep, it's blow.
More likely oil than water-coolant, because oil can withstand engine operating temperature without vaporizing off quickly, while water-coolant can't. But... not leaking from the plug hole. Leaking from ABOVE the plug hole, from the valve cover or a crack in the head (also not likely, since that area of the head doesn't carry oil under pressure, it carries "waste" oil under no particular pressure back to the crankcase.
Let's get that area cleaned, let it run for a short while, and pull that plug for another close examination. A little white paper (toilet paper or paper towel works well) will point out the highest moist spot in that area.