Welcome RevRider. I would agree with your assumption. I have a 2001 and it calls for 5w-30. I am of the same opinion as you that 5w-30 would be best for the engine. It will not void the warranty or anything like that if you use the 5w-30 and I don't see you getting a big plus mpg more because you are using the 5w-20 oil. You are right that all the chrysler product engines (EXCEPT the 3.5L engine - 10w-30 oil) are going to 5w-20 oil.
However if you have a vehicle with the Hemi engine in it, you need to use the oil they call for because of the Cylinder Deactivation factor that decides the amount of cylinders being used. A heart of the deactivation is the lifters and THEY SAY using 5w-20 oil is a must.
Avatar: EconoVoyager Concept
2001 Dodge Grand Caravan Sport 3.3L
2004 Chrysler Pacifica AWD 3.5L