I have a spark plug that has oil leaking into it, would this just be a simple O ring or a whole gasket, or seal?
And If I fix it would I notice a diffrence in the way the car runs?

 

Are you working on a 4 cyl engine with the spark plugs in the center of the valve cover? If you are the valve cover gasket needs to be replaced which is an easy job to do. The valve cover gasket set should come with the four rings that seal the spark plug holes.

 

Oil leak in the spark plug can cause the coil to fail/short. Having a new valve cover gasket or new o-rings can isolate the direct-ignition stalks and spark plugs from the oil in the valve cover area.