Just to throw a monkey wrench into the "thinner vs thicker" argument that is sort-of going on here....
In the owners manuals for many diesel trucks, they specifically require the use of either 5w40 or 15w40 HDEO (depending on climate) when the truck is to be used for any towing/hauling etc. The owners manual does specify the use of the various 30 weight offerings in HDEO oils when the vehicle is not used for any "load" applications.
Translation/thoughts:
The diesel engine manual analogy is not entirely transferable to gasoline passenger car engine applications, I'm sure that part of the reason for the heavy oil in the diesel has to do with higher particulate accumulations in the oil (diesel produces "soot" when burns, and it inevitably gets into the oil).
However and also: A harder working engine will cause higher oil temperatures and more extreme hot spots for the oil to deal with. A heavier weight oil is necessary to offset the effects of temperature and maintain the same effective viscosity and part separation in critical engine components, like the main bearing. Also, the higher weight oil will often have a higher flash point, further reducing oil "break down" in extreme applications.
Now, we already know that many 5w30 dino oils will shear to a 5w20 in short order, so running a 5w30 dino is pointless if you are trying to achieve some greater level of protection over 5w20. More importantly, one must ask if their engine works hard enough to justify the use of a heavier weight oil. I think the "thin vs thick" argument is not one with any single answer, instead, a smart vehicle owner might take a moment and consider their driving habits and climate and make an appropriate decision about which weight to run. There seems to be this consensus that engines are "engineered" to run a certain weight oil. I feel that this is an inaccurate assumption. For many decades, we have seen car manuals with charts that show the entire range of possible oil weights that one can purchase and use in their engine, each oil weight will have a "bar" on a chart that covers a range of climate and driving conditions. I've seen several of these charts include everything from 5w30 to 20w50. Nowadays, those charts can only be found in the manuals for the same vehicles sold outside of the USA. The same engine that is in my vehicle can be found in several vehicle models in Australia. Mine calls for 5w30, the Australian version calls for 10W40 or 20W50 depending on intended application. Several other vehicles I own or have owned in the past had the neat "charts" with oil weight applications.
Look at the larger picture and make an appropriate decision for your situation. For most people, 5w20 is absolutely perfectly fine. In fact, many engines would probably last just as long on straight 5 weight with the way most people drive.
-Eric