| If the cluster truely is a new part then the mileage should be blank. Regardless of what you do the only proper way to replace the cluster is to have it coded by the dealership or someplace with dealer quality software. this way the mileage can be transferred fromt he LCM which stores the mileage redundantly with the Cluster. What is also very important is that the ZCS (central coding key) which houses the list of all options installed on your vehicle is also transferred to the new cluster which can only be done during coding. if the cluster truely is new then this shouldnt be a problem.
Also there are 2 different clusters available for the e39 regardless of the model (528 540 whatever) there is a high version and a low version. High version will have a matrix display and will give you check control messages in text format. The low cluster has a pictogram of a car on the left side and gives you check control warnings by displaying a symbol on the pictogram of the car. | 
