Oily Sparkplug

I have a 1.6L Sentra and after a tuneup the car ran great for a few weeks. Then my gas mileage dropped and I started to loose oil volume and the remaining oil is black and dirty. I opened the hood and the cap for the in the valve cover was about 1" hanging out!
I tried to snap it back in but it wouldn't snap. I looked in and my spark plug had come apart and the top nipple was stuck in the wiring plug. The spark plug was also covered in black engine oil.
I replaced the plug and fixed the wire and the engine runs smoother now,but I am concerend that this may be due to a cross thread or something like that.

Will boths sides of a spark plug get oily when the cylinder is not firing properly? Or is this something much worse? I just can't imagine how oil get around the back side of the spark plug.

you mean theres oil on the ceramic part? or the electrode part?
if its on the ceramic part, no worry, its just a leaking grommet. a new valve cover gasket set should come with new grommets also. if its on the electrode, it could be a number of things. anything from a blown headgasket to blown piston rings, to bent valves, cracked valve guides, bad plugs, bad plug wires, distributor cap or rotor, you get the idea.

It's probably just the gromet then. That makes two seperate issues with the cap (on the ceramic side) of the plug comming off and a leaky gromet. The valve cover seal that I installed actually did not come with gromets.

Since I noticed the problem my car leaks/eats about a quart of oil a week. It couldn't have been all going into the spark plug hole so I am curious where is is getting out. I will have my mechanic look under the car when I get the oil changed.
What is really odd is that like 4 days after an oil change my oil was already dark and dirty....Can't be good for my PCV valve.