There shouldn't be oil in the spark plug recess. If there's oil there it has either trickled down from a leaking rocker cover gasket, or has been blown up from the piston due oil finding it's way into that piston bore..
If the spark plug seat/thread is damaged on that particular cylinder it will allow gases to escape - collect and 'bake hard' around the base of the plug - and form that 'gasket' you mentioned. If the seat - the thread which holds the plug is damaged (usually caused by overtightening a plug) a helicoil insert can be fitted.
An oiled plug/seat can cause a misfire.
However, there should be no oil inside the cylinder. If that plug is becoming 'oiled up' .. there's oil finding way onto the plug from within the cylinder, that suggests a piston ring problem. Oil is finding its way past the piston rings..
It's unlikely that worn valve stem seals or worn valve guides would allow much oil to find its way into the combustion chamber. Worn stem seals/guides are usually characterised by a cloud of blue smoke at start up from cold due to a dribble of oil having found its way into the combustion chamber whilst the car has been standing overnight.
Your misfire is probably caused by the carbon/oil build up at the seat of the spark plug. You need to find out where the oil is coming from.
Any half decent workshop would be able to give you an opinion within a couple of minutes. It's maybe worthwhile asking ...