ash &*(^s everything up. this includes main bearings (VERY soft bronze) as well as cam bearings (some alloy i know nothin' about). it makes no difference for rings. so, no ash is best (i.e. only mobil 1), and low ash is almost as good (some valvoline petroleum-based non-synthetics, but since you are required by bmw to use synthetics, this is of no relevance).
zinc: some people (castrol, others) add zinc to prevent high temperature scoring of the cylinder wall. this usually entails overheating the piss out of the motor, and really does not do a good job anyway. the cylidner bore gets scored, cos the rings do not catch up to the piss poor sheer characteristics of the oil. i take this as an indication of failure on the part of the oil designer. if sheer flow characteristics were done well, zinc would not be necessary to compensate.
more on sheer: this is critical in thin film such as cam bearings, or cylinder wall. it is interesitng to note that the viscosity rating as indicated on the bottle has absolutely nothing to do with it. i.e. a 20w50 oil can become like water (0w0 or worse) in thin film sheer. some people (like redline) study this stuff cos they sell the oil to racers, who do it at revs exceeding 10k. so they know what works well.
i change my oil at around 7.5 k, and ask bmw dealer to use my mobil1. since they do it for free until 48k, makes no difference (i am out 7 quarts x $5 per quart). i DO NOT reset the oil counter, cos this just creates unnecessary questions from the stealer (so when did you last change the oil ???).