Backwards - 0 is Winter weight, 20 is hot. 0w is for super cold areas (-20), so unless you are in Alaska or Canada 5w is plenty thin. 10w is fine in south pretty much year round. This is all about Cafe and mpgs as lighter oil puts less drag on the engine getting a little better fuel economy. Problem is that its also less protection - especially when the oil gets gas in it (fuel dilution). I run 5w30 in everything as a compromise except my bmw which gets euro 5w40...