All C oils are classed as long drain. The C3 oil is "thicker" (3.5 value at HTHS compared to the 2.9 for the C1) but it will be too old skool to claim that this helps the oils to last longer. Maybe that is the reason they moved to the C3 oil to reduce the effects of the insufficient design on the oil circulation system to cope with the dirty oil. Instead of fixing the problem, they use the "thicker" oil.
Good for the local dealer to use the C3. Have they figured out what 6 you have? They may service new 6s but this does not mean that from now on they can use the same barrel for all cars.