What causes oil in the spark plug tube in a 1999 Camry?

Spark plug tubes are threaded into head. Oil leaks by these threaded tubes at the head, which are under the cam (valve) cover. Remove threaded tube, reseal tube threads with thread compound (many good products on market, i.e., Permatex,et. al.) and re-thread the tube back into the head to proper ft. lbs. tightening. (29 ft. lbs. on 1990,1991, for example) There is a tool that will do this job without removing the Cam Cover, Assemacher Spark Plug Tube Remover (TOY150).

Leaking valve cover gasket leaking valve cover gasket.