I agree 5W-20 is what you want. You don't want to exceed 20 weight, as that is the weight the oil becomes when hot. So unless you live in a part of the country with extreme cold temps, 5W-20 is fine. Your car will not suffer from using 5W over 0W, as this is the weight the oil is when cold. Once the engine starts the temp changes the oil rather quickly past 0W.
I really don't know what constitutes warmed up oil (50F, 80F, 120F, 150F etc). I have both an oil pressure and oil temp guage on my insight. I can say that it takes a good 20-30 miles before my oil reaches the normal operating temp (180F at highway speeds). The water temp can reach normal within a mile along with autostop but the oil temp is a slow climb. Just a observation which may or may not be significant.
That's very significant in all cars. That's why lots of short trips need more frequent oil changes because the oil isn't able to get up to temperature to burn all the water and other crap off. It's also why you shouldn't rev very high until you know the oil is warm (or close to warm) because thicker oil provides much more protection. From what i've read, the 1st number is the viscocity at 0 degrees Celcius and the 2nd one is at 100 degrees C. A 0 weight oil gives great cold start capabilities since it's easy to pour but doesn't provide the needed high temperature protection that a 20, 30, 40... weight oil does. The Insight uses 0w-20 because the engine really doesn't run very hot and they can get away with running a 20 weight oil where other care would burn it up.