Safe oil change intervals are highly dependent on type of use. Short drives without fully warming up the oil or extended hard driving with high oil temps. obviously reduces its safe life. The X3 35i does have a fairly large oil cooler and does compute projected change intervals based on usage, but the possible 10 to 15K mile changes are generally thought to be too long if you are planning on keeping the car for well beyond the warrantee period.
For my track car, which does not accumulate many miles, but does see some engine "stress"
I do changes every 2K (Mobil 1 0W40) or once a year and always send the oil to Blackstone Labs <http://www.blackstone-labs.com/> for analysis.
For our X3 35i, I'm planning on an initial change at between 5 and 6K with an analysis to determine how much life the oil has remaining and how much metal wear has occurred. Of course, always use either BMW oil, German Castrol or Mobil 1 as recommended. The people at Blackstone Labs really know about potential oil failure and are a good source to use to find out if your change intervals are too often or too long.
Other N54 and N55 engined BMWs, which are driven fairly hard, seem to be giving fairly nice oil reports (low wear and low oil breakdown) at 5 to 7.5K (mile) change intervals, so this seems to be a safe distance.