from my understanding, the first number is the oil viscosity (weight) when cold, and the second is the viscosity (weight) when at operating temperature (According to SAE classification 0C is cold and 100 C is hot. so 5W30 is 5 Weight when @ 0C/32F and it's 30 Weight when it's heated up to 100C/212F - Oils sure have changed since the 70's.)
so you oil will only have 'lower' friction when cold..
If you want to save gas, use a lighter foot and you'll see a decent improvement. Using 0W30 will probably damage oil pump. if it says only to use 5W30, then you should probably only use 5W30. I have a Nissan sentra, and it has a chart in it telling you what oils to use..
you asked for 2 cents, and you got 4..
more infomation on oil viscosity:
Oil Viscosity
