I'm with hoff! Rocker gasket leaks, I think are due to blockage in the crankcase ventilation (PCV?). Once these do completely block, it forces a build up of pressure and eventually the oil has to come out somewhere- usually through the rocker cover & down onto the plugs. Had it on my 240 & simply changed the gasket the first time, surprise surprise it leaked again in about 500miles LOL!
Keep thinking about checking my 480, but I'm always reluctant to fiddle too much for fear of ballsing it up
Anyway good luck.