First, look at what oil does. It cools, cleans, scavenges and lubricates. The ability to cool is negligible for most of us, although thinner oil will theoretically cool easier. Viscosity really has nothing to do with the ability to clean (detergents, buffers, etc.) so we won't go there. Scavenging a little more so but not important enough for this post (the ability to hold contaminants in suspension to the filter medium). Lubricating, wow, where do we start? Oil forms a fluid wedge between the friction surfaces to prevent metal to metal. The tolerances between these parts are much smaller today than they ever were due to materials available, forging/machining techniques, and the R&D implemented in the engineering. The use of thicker oil is more often than not unnecessary. It causes power loss, elevated oil pressure, and decreased fuel mileage, given that they are used under the same conditions. To tell you the truth, I haven't owned any vehicles that didn't lose at least a quart of oil between changes. That's everything from my Corvair, El Camino, Prelude, Integra, assortment of mini-vans, to my current cars, an Expedition and of course, the left handed Aurora of death! The problem is finding a happy medium. If you live in Yuma, you may have to use a 20-50 oil to maintain the proper oil pressure. If you live in Seattle, you can probably live with a 5-30. A previous post described hearing valve clatter. Is that because your oil is too thin for your region? You may have to use a heavier oil due to your local conditions. People also use heavier oils in worn engines to fill the increased tolerances from wear. Sometimes, a heavier oil is used in smoking engines because it doesn't leak past the valve seals as readily. There is no one standard that can be applied to every car in every region. Usually the only time you will experience low oil pressure is at idle. If memory serves correct, my 327 in my ElCamino used to idle with less than 10 PSI. I'm happy with a 10 PSI per 1000 RPM rule. Thinner oils are happier than heavier oils being pumped at low temps so don't be surprised to hear some valve clatter upon start up if you live in Minnesota and run 20-50 in the winter.
BTW, I use 5-30 in FL w/ 95+ temps. I still have great oil pressure and I don't lose any more than if I were using 10-30. It's also very rare for my coolant temp to go above 200 degrees.