There is one good reason to use non approved oil, and that is its good for your engine! When BMW approves an oil, it compromises engine longevity with CATALYST longevity. Now guess which one government regulations require them to warranty for longer? Yup, the cat. Most manufacturer specified oils are good for cats and recently mileage and not good for the engine. Look at the latest oil standards - SN, etc. The is the crap ford is using 0W20! Obviously 10W60 is not MPG focused oil, the M division has a say here. But that is why I would consider using the boutique NON approved oils, as they are likely to be good for the engine and eat your cat over 50,000 miles. But many here run cat less.
I apologize for spelling mistakes up front, they are a result of multitasking.