Correct oil weight for E320

I have a new E320 and asked the dealer about synthetics and was told they don't recommend it because of cost. I figure it costs something under $.002 per mile to use it versus regular oil. In any case, the dealer uses 20W-50 all year. I''m in Tucson, AZ where winters are lows inthe 40s and June-September are highs in the 100s. Even for here this seemed a bit heavy for a newer car so I called another dealer in Phoenix (same climate, maybe hotter), who uses 5W-30.

The MB manual has no info I can find on recommended oil weights and nothing about synthetics. Any input appreciated. In my other cars I have been using Mobil 1 0W-30, per Mobil's recommendation, for cars that the manufacturer says to use 5W30, and changing oil on the car manufacturers schedule (7500 miles).