IF you do a lot of short trips and build up condensation in the oil (ie the oil temp never really gets too hot) the oil level gauge can display a false oil level (ie low). Read this TIS (oil condition sensor - page #5 bottom of page)...
BMW_oil_condition_sensor.pdf
These TIS's might also be of interest for any early build cars.
SI B 12 05 07 Low Oil Level Warning Message is Displayed.pdf
SI B12 18 08 N54 Oil Level Programming.pdf