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The maintenance guide BMW would give you, is horseshit anyways. I wouldn't ever recommend waiting 15k mi to change the oil. Especially the garbage Castrol oil BMW uses.

What BMW tells you to do for upkeep is the absolute bare minimum to keep the car running. I suspect their engineers were told to make the plan based on a vehicle being leased for the duration of the warranty period. After that, they don't care. In fact, their plan probably ensures that your vehicle will be in their shops more often to get more money out of you in repairs.

If you plan on keeping your car until it dies, plan on doing more than what BMW recommends. And start doing it long before the 4/50k warranty expires. There's also other things which BMW doesn't say needs to be done, but should still be done in order to get the most out of your car.