When changing the gearbox oil, you not only give the gearbox fresh oil, you will also have the opportunity to clean out all the small metal fragments that have been created inside the gearbox and collected on a small magnet on the drain plug.
The easiest is to simply follow the SI (Service Interval) lights indicating Oil Service or Inspection. When maintenance is to be done, the yellow LED will come on together with either the Oil Service or Inspection indicator. If the maintenance is not made, the red LED will also illuminate.
The BMW E34 is the version of the BMW 5 Series automobile sold from February 1988 to 1995 with the Touring model ending June 1996. It replaced the BMW E28 in 1988 and was replaced by the E39 in 1996.