usually on first trans fluid change that "grey goo" is common. It should be done every 30k but in later years BMW promoted "lifetime" fluids & longer intervals which equaled more service repairs to auto trans'. Just get into routine of doing it every 30k and it'll be good for a long time. Maintenance on a regular schedule is beneficial to ALL BMWs. They're not built sloppy so a regular maintenance schedule is critical to it's longevity.