Ford started using 5W-20 around 2001 in the Focus and Honda has been using it widespread since around the same time. Even the CRV uses 5W-20. My dad told me their 2006 CRV says 5W-20 right on the oil cap and I couldn't believe an SUV would use that light of an oil. But much to my suprise when he popped the hood, there it was in yellow.
Some say the switch to 5W-20 from 5W-30 has more to do with a marginal increase in fuel economy to satisfy CAFE standards. I know Dodge and Chrysler started using 5W-20 over the last few years in the Stratus and Sebrings with the 2.7L V-6 engines, whereas previously that motor used 5W-30, like in the 2001 Intrepid I used to own.
From everything I've read up on, the MPG difference between 5W-20 over 30 is very marginal. One thing I did see though was at BITOG, and seemed to indicate 5W-20's look like they have a higher content of moly in their mixes over a 5W-30, so perhaps to make up for the thinner weight the additives were beefed up a little bit.
Yep - not much difference. Use whatever is on the oil cap and/or owners manual.