Hi, After a quick look the parts book and the manual they don't show washers but a copper washer would seal it. However the fact that oil or oil fumes are appearing indicates a breather is blocked if indeed there is one, again that doesn't show unless it has a gas permeable bolt. Perhaps Harvey can confirm or deny. If you do use copper washers and seal it it could well leak somewhere else. My suggestion would be to drill a small hole in the filler plug and put a loose fitting split pin in and tease the legs apart, this will stop most crap going in and let any pressure out.
Colin