hi, check to see if the plug is firing first. remove the offending cylinder ht lead and plug. reconnect the HT lead to the plug and rest it on the rockerbox cover to earth it. fire up the engine and see if the plug is sparking-DONT TOUCH IT WHILE THE ENGINE IS RUNNING! If it sparks you know its either cos of the oil or its because of petrol. check the injectors by swapping them over and see if this helps.
the oil shouldnt get into the plug lead top - its almost air tight (or should be), so should have no affect on the conduction of the plug electrically.
To be honest its likely that if the car's been ragged in its life (being a sporting!) you'll have scored bores and oil is seeping up from under the cylinders. Mind, the head gasket life is supposed to be 60k miles so you might be overdue a change. At worst, you might have a cracked cylinder head leaking lots of oil into the cylinder.
Steve's idea will cost you about �6 to replace the rockerbox seal...hope any of this helps!