My car idles a LOT. Take two kids to two different schools, sit in the drop off line idling for quite a long time. Pick them up, lots and lots more idle time. Like 3 hours of idle time every single day. Times five days a week. (Yeah police cars and taxi's idle a lot more, but THIS is my car that I maintain and care about)
I also read reports of stretched timing chains using 0W20 and slightly longer drain intervals.
I also understand CAFE is caused due to gov mandates. And that if the actual engine engineer was in charge of setting the oil recommendation, he is likely to recommend 5W30 or 0W40.
Now do I run the factory 0W20, or bump it up to 0W30/5W30? Idle time adds fuel dilution.
In the grand scheme of things, 0W20 is OK most of the time. But it also has less margin for error. If the engine overheats, you will be glad to have a 30 weight in it. If you are towing heavy loads, you will be happy to have a 30 weight in it.