One thing about oils is that you want to use the LOWEST viscosity spread reasonable. There are more additives used to get from 0 at cold to 30 at op temp than 5 at cold to 30 at op temp. The more additives in the oil, the less its life is (except the oil that is designed for extended drain intervals - whole different ballgame).
What does this mean to us? Since we are all pretty religious about changing our oil when it is due, probably not much. If you're changing Mobil 1 at 3000 miles, it means nothing. But to be on the safe side, 0W30 should be used when the climate calls for it and then switched to 5W30 when appropriate (or 5W20 if you like, but its not for me).
