I started reading on wikipedia about oil viscosity http://en.wikipedia.org/wiki/Motor_oil
I always thought it 5w30 stood for 5 "weight" 30. But what it really stands for is Winter. According to wikipedia
"Based on the coldest temperature the oil passes at, that oil is graded as SAE viscosity grade 0W, 5W, 10W, 15W, 20W, or 25W. The lower the viscosity grade, the lower the temperature the oil can pass. For example, if an oil passes at the specifications for 10W and 5W, but fails for 0W, then that oil must be labeled as an SAE 5W. That oil cannot be labeled as either 0W or 10W."
That's quite informative. I guess paying for 0W oils is only a good idea if you live somewhere that gets quite cold. The lowest temperature I have seen in the morning is 27 degrees F.