Just did an oil change on a new to me 128i there are black small rock like chunks that are magnetic that came out with the oil about 6mm, pretty nonuniform no smooth sides, The car is a 2008, could this be carbon + shavings, or casting remains?
casting flash would be long gone by now small bits of carbon/dirt are pretty normal. I wouldn't worry about it too too much but keep an eye on things.
What oil are you using and whats the drain intervals at? I suggest 5w40 and change every 8k. Pull the vanos screens/check valves and clean those as well.