I am a new 6 owner approaching my first service (down to 1 green) and need some oil change advice. (The car was fully serviced just prior to my purchase) I have heard that there is something you can add to your oil prior to your oil change which helps get all the gunk out. What is it and does it work on our cars, any opinions welcome. Also what is the best oil recommended by the group. Synthetic - I have heard it can cause leaks. Any opinions welcome. My car is a 1989 M635 Auto in Oz.