Obviously I know nothing about BMW,mostly pretending that have some knowledge,or 20 years experience with BMWs.

From Pelican parts:

" The transfer case fluid condition is monitored by the transfer case control module. In theory, you will be notified when a service is due on the info display via the transfer case control module. "

BMW E90 Transfer Case Fluid Replacement | E91, E92, E93 | Pelican Parts DIY Maintenance Article