What I mean is using Mercedes genuine parts is a good practise. There is always a better quality material used, etc. For example full kit to change the gear box oil is around 130 quid from Mercedes (7 ltr of oil, filter, gasket, electrical plug and red clip).
Once I have used Mobil oil for the box and it died in 2K, that was a recommendation from a shop (don't go to Mercedes, too expensive thay are using mobil and you only pay for star sign),after they didn't want to talk to me, said it was my mechanic fault.
I would change oil again to see if it helps, will defo not do anything bad to the car. My mech charges 30 quid to change the oil - not that expensive in the end all in.
For yours it is a very low mileage, mine had 120K oil never changed and it started to had those symptoms.
Since using genuine parts never had a problem again.