I am an advocate of synthitic oil. I have never heard of anything bad about the wide ranges. I would feel more comfortable if you were talking about Mobil 1, however.
Synthetic is just better oil molecules that take longer to break down. Even the heavier weight, 5W-50, is not that "heavy" as compared to regular 10W-40. In Vegas, temperatures in excess of 100F, you absolutely want something that is more prudent in warmer climates.
My manual recommends 20W-50 for those high temperatures. I'd recommend Mobil 1. Don't look for cheap deal when buying oil for your Benz. Cheap deals usually compromises longevity.
Just my 2 cents.
'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)