Unfortunately, the noise you are describing is a collapsed lifter.
If you can isolate it to one cylinder bank, you can pop off the valve cover, and see if you can locate it.
If you are mechanically inclined, you can even replace it yourself, as it isn't an overly difficult procedure, if you are used to working on cams and valves.

As for the advice you got to switch oils, the oil you had was perfectly fine.
5W-40 engine oil is recommended for the engine, as long as it meets the MB 229.3 or 229.5 standards.

There is NO difference in the protection that 5W-40 oil provides at operating temp in comparison to 0W-40, 10W-40, or even 15W-40. The difference between these grades of oil is only when the oil is COLD, when it is thick.

Unless you live in Minnesota, and its well below 0 degrees when you are getting ready to start your Crossfire up to drive to work, you won't see any difference between 0W-40 and 5W-40 from a protection standpoint.

BC.