The Max line is the absolute maximum level the oil can be topped up to. As I said in a previous post as long as it is below the max line but above the minimum when hot you are fine. The 'sweet spot' for want of a better word is the midway line (halfway between min and max). After it has been serviced this is where I would expect to see the oil level sitting at. This has been the case for both W205 diesels I've owned. I'd say your GLC has hardly used any oil in 6000 miles if it is sitting midway when hot and on a flat surface.
To be honesI never looked at it until now as my 205 never had a display but a dip stick (c class ) so was a bit of a shock but I will see at service where they put the level
Magpiemale