Guys, it's mostly Ford and Honda that spec 0/5W-20 oils ... and I don't think they redesigned a single engine with this thinner oil in mind. All the engines that call for it are older designs with the same clearances as when they called for 5W-30 (like Dad's Mercury Grand Marquis with its 4.6L V8).
I've seen dozens of UOAs (Lab tests) using 5W-20 and they usually look superb. I'd use 0/5W-20 without much reservation. Just don't go over 5,000 miles without doing a UOA from a place like this:
http://www.blackstone-labs.com/gas_engines.html