Oil in the spark plug well will foul the spark plug and that is why your are getting slight missing.
Run a small piece of Bounty all along the valve cover gasket when the engine is hot and you smell the oil burning. If bounty is soaked with oil, redo the valve cover gasket and you should be fine.

Spark plug wear so fast- Most likely wrong gap or Low heat transfer plug and stop and go driving or High heat tranfer plug and Highway driving. I don't know you tell me?