I assume it is a Ford based on the code you provided.
It means that there is oil getting into the plug galley for that plug, most likely due to a leaking valve cover gasket. There are o-rings that surround the plug galleys on the usual Inline 4 cylinder Ford motor that often go bad and let oil drip into the galleys where the plugs are located, eventually leaking into the threads of the plug and into the cylinder just like penetrating oil would.. The oil will contaminate the plugs and wires and cause misfires. Check the valve cover gasket for signs of leaks. Stick your finger into the plug galley, if it comes out with oil on it, replace the gasket.
Code po303 is Random engine misfire, probably being caused by the reason I listed above. Also, if you did not change the wires, make sure you do, especially if you had oil on the plug(s).
I doubt it is something internal, chances are it is something as simple as the Valve cover gasket leaking, so dont panic yet.
Note: If your piston rings were bad you would be blowing a ton of blue smoke out of the tail pipe and have a serious compression issue.
Update: Based on the additional info you provided, replace the valve cover gasket anyways, and replace the ignition wires, clean the new plugs with come carb cleaner before putting them back in to make sure there is no oil on them. Clear the trouble code, drive for a few days. If the same trouble code comes back, do a compression test on the cylinder that has an oil contamination problem. Also, run some injector cleaner through the fuel rail (or have a garage do it). Dont use the stuff you put in the tank, it will just stir up the dirt in the tank and complicate things. Use the propellant based cleaner that gets injected directly into the fuel rail. A dirty injector can cause a po303 code as well.
Never jump to the more severe solution when simpler solutions are often the answer.
Have code po303 changed spark plugs only spark plug three has oil on it what does this mean?
Oil or fuel? Could be bad plug, wire, cap, rotor, or coil pack, depending on car. Have to know make/model/year/and engine to help further.
Have code po303 changed spark plugs only spark plug three has oil on it what does this mean?
Piston rings are probably bad. But if you're lucky, it may just be a bad PCV valve.
