HI i own mercedes 1980 300d small amout white smoke caming out from oil fil?

was there any creamy coloured oil on filler cap? if yes, then possible your head gasket on its way out. if the wet stuff on rocker gasket was water, same answer about h/gasket. if its oil then change your rocker box gasket, and check oil level. not to sure about white smoke, but if h/gasket on its way out then that could be the answer.

hope it helps
September 2010