Bmw 635D - water in oil , condensation .

Remove your oil cap and check . If you see condensation it means you did not drive the car enough to reach optimum temperature and it made condensation.
Change your oil at 5000 miles at most and also before entering cold season.
Short trips damage oil very much. Also city driving . If you start car and drive 5 miles and stop and drive again next day is why the condensation appear.
This is a grand tourer, not meant to drive 5 miles in second gear. Is meant to cross continents. Take it to open road and drive it. Let it warm up and push it a bit. Is a sport GT not a smart.