I just changed the spark plugs and notice thick oil on the outside of plug 3 and 5. Thinking the valve cover might be leaking, I replaced the gasket and drove 50 miles. When I pulled 3 and 5 again there is new oil on the plugs (it is again thick and black). Any opinion on whether the head has a crack or two allowing this oil to seep or might something else be going on? If there is a crack, is this reparable (I assume that the head must be pulled for this)?