I generally do mine every 10 or so. If you haven't replaced the oil spray bar bolts with the new loctite-ed bolts, it's a good time to do it, or at least check that the bolts are tight (only about 10ft-lbs, I think the spec is ~80 in-lbs?).
Take a crack at it yourself - it's really a peice of cake--
BMWe34.net Link--
Originally Posted by Greg