1) Shell Helix Ultra 5W40 is on 229.3, I don't see Motul 8100 on there - how is that "changing according to M-B specification...."? (Altho I suspect that oil is fine, many oils not on 229.3 / .5 are fine.)
2) Depends on where you live (the climate) and how you drive the car.

With the 0 weight oils, your operating-temperature oil pressures will be around 3 to 5 pounds. That's way too 'light' for my comfort level. 5W40 will raise idling pressures at operating temperature to 10 to 15 pounds.
Pressures at 3000 rpm will go from around 20-25 pounds (0W40) to 40-50 pounds (5W40). That makes me way more comfortable.

A switch to 5W50 will give a less-impressive lift to pressures, but in a COLD engine, will mean idle pressures over my gauge's top end of 100 pounds - that bothers me. I did use 5W50 back when I was putting so many miles on the car that I was doing 2 or 3 changes a year - AND driving the car hard (wide open throttle acceleration, pressing to the red line on the tach, etc.); those days are essentially over.

I preach 229.3 and 229.5 but I no longer preach that ONLY those oils are OK. I instead look at the recommendations and draw conclusions about what MB engineers are saying 'between the lines'. And if you look, you will see, they emphasize 5W40 for most users. 229.3, back in 2009, emphasized 0W40. The trend here is to higher viscosity. One thing does not change: Full Synthetic is what you want. That leads me to this conclusion: For most of us, 5W40 full synthetic is what you want; buy the brand you like, if it is not on 229.3/.5 you void the warranty - but that warranty expired years ago, so who cares?