See also:

Vehicle data, GL 550
The data stated here refers specifically to a vehicle with standard equipment. Consult an authorized Mercedes-Benz Center for the data for all vehicle variants and trim levels. ...

Setting the side air vents
•► To open/close: turn thumbwheel 3 up or down. Side window defroster vent 1 is never completely shut, even if side air vent 2 is shut. ...

Additives
Do not use any additives in the engine oil. This could damage the engine. ...