what model of Benz do you have that takes only five quarts?
My C's take very near the six and one half quarts that is the crank capacity when I do a change.
Did you know that the mercedes crank case can have thei oil withdrawn through the dip stick tube?
I change my 15W50 every 4K to 5K miles.

Those fill for life transmissions seem by many to enjoy fluid changes every 50K to 60K miles for longevity.

Seems like a new car for you that is why I mention it.