mate the reason i change my oil at 10,000km (6,000miles) is fairly simple and you could say throwing money away, however a long time ago i was told that if you want a diesel motor to last then change the oil frequently, now i dont know if this is true or not, but if i way up the costs of the extra oil changes against a potential turbo or even replacement motor ( please remember i,m in australia, and we pay about three time what you do for parts etc) then i call it cheap insurance.

i know with my petrol engined boat that it gets an oil and filter change once a year no matter how few hours it does, generally speaking some where between 30 and 50 hours a year, so if you work that out at driving flat out in low gear up a hill at 50 mph thats 1500 mile only, so running your car for 15,000 to 20,000 km between oil changes to me seems like inviting trouble

anyway thats my reasoning only, obviously others may have differing opinions and they are entitled to them, however i very much doubt that BMW cares what you do with it once the warranty has expired