BMW run the crankcase at vacuum, there is a vacuum pump specifically for this reason. Leave the oil cap off, or if there was a dipstick, not fully inserted, and you will get a check engine light, and it will run terrible as you have a huge vacuum leak. When a front or rear main oil seal fail, the first sign is noise like a moaning as air is being pulled INTO the crankcase, they won’t leak oil while running, only after being shut off. Take off the oil cap and the noise will stop.
There is no manifold vacuum because there is no throttle body to allow vacuum to form.