And how do you explain Ford & Honda using 5W20...?I would stay far away from anything xW30. What the hot climate basically means is that the second number is far more important than the first.
The first number is the viscosity at 104*F, the second is viscosity at 212*F. In that kind of climate, you'll be very near 104*F right when you start up, and you'll spend most of the time closer to 212*F.
According to the chart in post #2, MB doesn't think xW30 will reliably keep bearing surfaces in the engine from touching each other in those temps.
If you do use 5W30, I think you should use a much narrower rev range... use manual mode, and keep an extra 500 RPM away from lugging and 1000 RPM away from redline...