This topic is a HUGE
can of worms. On the one hand is the camp that says use the factory oil while under warranty (the relevant BMW spec is LL-04), and to use anything else is a waste of money. In the other camp are those that like to study specs of oils, and feel they can find LL-04 oils that are superior to the factory oil (which is made by Castrol, by the way), and by using a superior oil are getting superior protection for their engine.
And that doesn't even scratch the topic of oil change interval, which is a whole internet war in its on right.
If you look in the maintenance/warranty area of the board, there are some incredible slap fights over this.