I don’t know all of the reasons behind BMW’s (and Mercedes’, and other European makes’) requirement for their Euro-spec oil, but I certainly wouldn’t take a chance on using anything other than an oil that meets their specs. I believe that their ridiculously-extended oil change intervals are the main reason for a special-spec oil, but there may be other factors as well.
Ultimately, I think the question is which engines will mate with its transmission, its motor mounts, and its electronics. My guess (subject to correction, of course) is that only a BMW engine would be possible.