I believe the oil can go the 15k with no issues but its the filter that breaks down over time. This a very worst case scenario (below link) but interesting. Does it relate to this discussion, probably not because we care about our BMW's but interesting never the less to know that folks out there are oblivious to regular oil changes.
Check this link out:
http://forum.e46fanatics.com/showthr...ighlight=0w-40