I run either Mobile 1 5W50 or Castrol Edge 5W40 - both are on the 229.3 approval list.
0W40 is just too thin for my liking, here in Georgia's heat.

If you take the car to an MB or Chrysler dealer, by default, it will get 5W40 - that is the current recommendation from MB.

BUT they did not do what you told them to do - so the free oil change sounds right to me!