That is a good one. You have to select some criteria, your mileage, ambient temperature, a guided selection or not, car still under warranty or not etc.
But I always new it: 10W 40 is the best for my car.
Give it a try and let me know what's the result. Maybe they recommend something different because you live in a warmer or colder area.
http://www.mobiloil.com/USA-English/.../Homepage.aspx