Bill,
People are getting all bent out of shape because the Mobil 1, 5w30 may shear marginally more than their 10w30. The bottom line is: how are the wear rates after 5000-6000 miles? You could switch back and forth between these two oils and you would not see any significant difference in wear protection. This is probably true for 95% of Mobil 1 users. I'd use whatever grade of mobil 1 you happen to find on sale.
If you lived in an extremely cold climate, you might do better with the 5w30. Likewise, in an extremely hot climate, the 10w30 might provide slightly lower wear. For most folks, it is not worth worrying about. I have been using Amsoil for 24 years (I've been a dealer since 1980) and I haven't seen any difference in wear protection with their 5w30 or 10w30 synthetics. I live in Alabama and I've primarily been running 0w30 or 5w30 synthetics in my own vehicles for the past six years.
Tooslick