changing plugs, have oil on the inside where it ignites

 

could be just a gasket that goes aroung the spark plug holes.

 

spark plug tube seals, failure of them is very common.

They are about $6 each, but you have to pull the valve cover gasket and some other things to get to them. To do both front and rears, I ended up spending about $65 in various seals and gaskets, though I still have yet to actually tackle it.