oil change

I have a 520d October 2008, touring. The dealer says oil change interval is 20000 miles. I find this hard to believe. Anyone have a similar vehicle ? The car now has 85k miles on it, and burns no oil between services. There was 46k miles on it when I bought it.

Car is due its first service shortly (30k miles from new) and its still under original 3 year warranty (expires end of may 2011), so should it be serviced in a main dealer or with my own non main dealer mechanic who also has diag equipment etc. I'm not bothered about full services history by...