I religously changed oil and filter every 3,000 miles on my 240D, which is basically the same engine with one cylinder chopped off. The original engine went 380,000 miles.
The way I change oil probably cleans the engine as well as a slightly more frequent interval. I come in on Friday night, pull the plug and filter while everything is piping hot and let it drain overnight. This gets all the sludge out of the bottom, and probably does the same as a 2,500 mile interval.
Change oil hot and change oil often,
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in