My daughter has a 99 DOHC neon. She has been going through some oil in it, no sign of smoke coming from the tail pipe. I decided to change the plugs in it to see if it would improve the mileage and the #1 & #4 plug holes had a lot of oil in them. Not inside the cylinder in the tube leading down to the plug. My question is what could cause this. Could possibly the valve cover gasket be leaking from the inside and running down in the tubes or what?