Q: I replaced 2 spark plugs in cylinder 5 to fix a misfire.

Leaking valve covers on this engine allow oil down into the spark plug recesses as you note. Besides shorting the plug, the oil degrades the boot portion of the coil which plugs onto the spark plug, creating other opportunities for a short and misfire.

By all means, replace the leaking valve cover gasket and, given your mileage, replace all the plugs and coils if the boots show any sign of oil contamination. Replacing only 1 or 2 plugs/coils doesn’t always cure misfire problems especially once oil is introduced. I would recommend having your valve cover gasket serviced by a professional mobile mechanic, like one from YourMechanic to start with. A certified professional from YourMechanic can replace the valve cover gasket, and can also perform spark plug and ignition coil replacement services if necessary.