I'm also in AZ (Tucson) and both MBUSI and Mobil told me that Mobil 1 10W-30 was the right choice in spite of the high temps. Mobil claims it protects as well as 10W-40s and doesn't require as much "viscosity adjusters" in the mix as an oil with a wider spread, like 15W-50 or 5W-30.
Between their advice and an 8 year extended warranty, that's what I'm using in my ML320 and E320.
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