First off: Wind chill ony affects things that are already warm... like people's bodies. A cold car in -10F in NOT going to be any colder because the wind is blowing!!!
Then the oil is thinner seeming when it's hot and if it was a lower viscosity it would be even worse. The 5W-20 is fine as someone said if you never run your engine very hard. I would NOT do it in an SVT. The 0W-30 as was said has a 'wider' span than 5W-30 and that does need more "stuff" in it to make it span that gap... so a little less of the oil is really oil... not much to worry about though. The "0" is only at startup, and doesn't matter too much... And yeah, viscosity is really a very complex issue, that is part of why it's said: use what is recommended by the manufacturer!!!! and do NOT use 10W-40 (unless you live in Death Valley... then it MIGHT be ok)
