What causes oil to collect around the base of the spark plugs on 1997 cadillac?

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this is a 1997 cadillac deville with the 4.6 liter northstar V8. it has 170,000 miles on it - mostly highway miles. has been misfiring recently & has rough idle. we just replaced O2 sensor & a defective coil. now runs better but smells like something is burning under the hood. Should valve cover gaskets be replaced? Also spark plug tubes? Thank You!!