Don't quote me on this but, I believe next time you change your oil, undo the 2 bolts holding the sender in, remove the sender, install new o-ring(s ), reinstall.
This is what the beast looks like:
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1R510G7PR1RR1AEIXE&year=1999&make=MB&model=C-230-002&category=A&part=Oil+Level+Sender

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2007 C 230 Sport.