Oil in spark plug wells is almost always caused by an old perished gasket. After a number of years the gasket shrinks due to constant heating and coiling cycles and from the contaminants in the oil. Oil like water and electricity always finds the path of least resistance and after some time, creates a big enough gap to allow more oil to flow in to the well.
Once your plugs have bathed in oil for awhile, they are pretty much done. And even if cleaned up should really be replaced. As the ceramic can no longer efficiently dissipate heat. As the oil level rises in the well it moves it way up to the cap of the plug. Oil however won't really affect your car by causing misfires, as all it might do is create a bit of resistance and oil doesn't tend to conduct electricity.
The main causes of misfires are electrical faults within the ignition leads or faulty plug. Again heat and cold cycles tend to cause the leads to break down over time, usually on the inside where you can't see it.
If the engine starts OK and runs when cold then starts missing and jerking. The ignition leads are most likely at fault. As they tend to be 'tighter' when cold but as the leads heat up, everything expands and that close gap widens, and the electricity needs to jump a wider gap, this can happen numerous times in a damaged lead. So in the end the spark at the plug is both weak and erratic.