As I remember, the service interval on my 2001 S203 C270 was 15.5k miles or 2 years, whichever came first - and the 15.5k miles was because she was on fully synthetic oil. I would imagine that your 2006 W203 C220 would be the same. The 2008 S204 C220 I now have, also on fully synthetic, is 15.5k miles or 1 year, whichever comes first, and IMHO MB changed things to get more work for their workshops - the change adds about £150-200 per year to the servicing cost compared to my S203!!