I have replaced the gasket (the new one will be softer):

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ426P0OE0JO&year=1983&make=MB&model=300-DT-001&category=All&part=Oil+Filler+Cap+Gasket

I also put a little grease around the lip of the filler, it seems to help the seal.