I found this interesting info on the Blackstone labs web site:
Which Viscosity to Use?
Engine owners often stray from manufacturers' recommendations regarding viscosity of oils. The engine builders dyno-test their engines using a specific viscosity oil, so when you use the viscosity they recommend, you are working with a known result. Going to another viscosity is an experiment, but it's usually a harmless one. For the sake of efficiency you want to run the lightest grade oil in your engine possible, within limits. We are seeing that trend for newer engines, for which the recommended grade is getting progressively lighter. The common 10W/30 has become a 5W/30, and some manufacturers even recommend 5W/20 oil. On the other hand, we can't see (in oil analysis) where it hurts anything to run heavier 10W/30s or even 10W/40s in modern automotive engines. The heavier oils provide more bearing film, and that's important at the lower end. If your oil is too light, the bearing metals can increase. If the oil is too heavy, the upper end metals can increase. The trick is to find the right viscosity for your particular engine, which is why we suggest following the manufacturer's recommendation.
Blackstone states that viscosity is not just the resistance/thickness of the oil, but the thickness of oil at a certain temperature and the standard temp.used to measure is @210F. Yesterday our temp down here hit 95F and my oil temp was reading 228, makes me wonder what I'll be seeing when summer hits hard and we have 100F+ days. If you read their analysis, Dodge is really running thinner oil to help fuel economy probably to make the CAFE/EPA green weenies happy.