The BMW factory is aware of the problems that 6-cylinder owners have been having with door locks freezing in the winter. The factory has published Parts Bulletin #5/71 about the problem and it is now possible to order a spray can from your dealer that will hopefully solve the problem. This is part number 51 91 9 898 827 (a new number), and it sells for about $6.50.
As an alternate, many people have success using WD-40 as a preventive measure in their locks.
Do not use silicone in the locks. It tends to build up with time and will cause difficult operation.
Author: Ed Vozek