Oil viscosity is what we are all discussing. The primary difference in oils is their purity and ability to cool, lubricate and suspend particles that contaminate the engine in order to release them in the filter medium. So a 5W20 and a 5W30 at cold start up have the same viscosity "5" once heated to operational temperature the viscosity modifiers in the oil change it to a thicker viscosity "20" or "30". This is also where the oil quality from one to another can make a difference. Higher quality oils will maintain their modifiers for longer period and withstand higher temperatures with less breakdown (i.e. sludge).
Now the viscosity at temperature is what we are all discussing. Higher viscosity has a better "cling" aspect then lower viscosity. It also takes more force to circulate a higher viscosity oil. Therefore a 30 weight oil will cling better and maintain a longer lubrication "cushion" and will quiet noisier engines, particularly when the engine has devices that modify how it runs that use the oiling system to vary its operation, i.e. cam phasers, lifters, etc. The counter to this argument can be that in today's engines with tighter tolerances a thicker viscosity can impact the oiling of bearings, etc. particularly at higher engine RPMs. However, as the engine wears and the gaps grow and tolerances start to move toward their outer/lerger limits higher viscosity oils can provide that additional cushion in which to ride out and in effect remove some of the "tolerance gap" caused by wear and age.
I come from having a Ford 5.4 3 valve and can absolutely stand behind the use of 5w30 for higher mileage engines and the fact that is does substantially decrease some of the "noise". I would even go so far as to say that when the "noise" level increases not going to a 5w30 may bring about a shorter overall life span for the engine and particularly those components of the engine that are manipulated by the oiling system.
As for the primary reason FCA is using 5w20, is the same as Ford, GM and everyone else.....tenths of a mile per gallon in fuel savings. I remember clearly in the very late 90s or early 2000 Ford particularly coming out and stating that nearly all gas engines dating back to the early 90s were good or prefered to run 5w20.
The interesting thing I'd like to know is why have they not moved to a 0w20 or 0w30? A 0 weight oil will get up in to the motor quicker and would in theory result in less "cold" start wear then a 5 weight.
