If u have to add 1L every 5K, your M54 burns more than normally. I'm sure you are not going to rebuild your motor, so i'll give you the oils that burn the least with M54.
Lubro Moly 5w40
Those 3 will burn the least. Mobil 1 and Castrol are donkey p!ss thin but they are great when you have a fresh motor.
1990 M5 Euro
You wash your car like it was your firstborn child, you tend to its needs like it was your own body, you protect it like it's your family, then you drive it like you stole it