IMO you're better off asking him as one thing I have noticed having come across to BMW ownership - no one seems to like deviating from what BMW state. And as for the reasons you state, I'm also dubious about whether we can improve on what they have recommended (ie they recommend the minimum requirement?) An oil expert will be able to advise you the best way to go
(btw this isn;t a dig at anyone on here, I only put in the Castrol stuff that BMW recommend This way of thinking is contageous )