I know a lot of people on here really have a hard time accepting 5w20 oils, but I think that if someone is doing short trips in cold weather, it will probably show better wear numbers than a thicker oil would. Think about it, if you're doing short trips, your oil never gets up to 100c, at least not for very long. So under these conditions if you run a thicker oil, you're gonna get a lot less oil flow.
Looking at the specs of a lot of 5w20s compared to the same brand of 5w30, the viscosity at 40c is thinner too, so all along the temperature curve you'll have an oil that is flowing better. (example, Castrol GTX 5w20's viscosity at 40c is 50, versus 63 for their 5w30)
So if you're the type of driver who drives less than 10 miles at a time in the cold winter months, you really should try out 5w20, even if your owner's manual doesn't specify it.
I am probably going to try out 5w20 next winter in my mom's 94 Probe GT, since she definitely falls into this category of drivers now. She used to pile on miles fast and do all highway when she was in sales, so I had her car running Mobil 1 10w30. But now she's retired and only drives in short 2-3 mile spurts, very rarely does she get out and do a long highway run. I probably should not have put 10w30 Mobil 1 in her car last fall now that I think about it.