i change oil when it looks fairly dirty on the dipstick , usually at 7500 miles , that is halfway between services .

1/2 a quart of ATF fluid can be substituted for those "special engine cleaners" you can buy off the shelf. With the engine warmed up, run it at idle or a little higher, parked, then do a normal drain and fill afterwards. Be warned though, even though unlikely since you're car isn't an early 90's vehicle, that a leak may appear due to worn gaskets. Either way, I don't know if you really need it, but if it makes you feel better, then by all means.

If you've got major sludge, then this will probably help. Google Images using the keyword engine sludge, these motors aren't bad compared to some earlier E39 540's I've seen in the past due to poor or lack of maintenance, using the improper weights, etc.

Killcrap does his every 3k, but at $80 my cost for parts vs his being cheaper since he works for the dealerships, I think 3k is overkill for the "normal" synthetic user. I usually stick w/ no more than 10k tops. www.bobistheoilguy.com has samples of BMW engines that had their oil tested, plus it gives lots of good reading if someone is really interested in this kind of information.