The manual for my Audi A4 says that I should use 0w30 oil, and when that isn't available, use 5w30. Naturally, I can never find 0w30 anywhere. In an effort to understand the advantages and disadvantage of both oils, I did some googling. Along the way, I found a nice, plain-english site describing everything practical you need to know about motor oil. Check it out at http://www.nordicgroup.us/oil.htm.
Wow, thinking about it now, I haven't crawled under a car to change the oil myself in almost a decade.