As others stated, check out Bobistheoilguy forum to learn about oil and weights. I dont think it has been said here yet that viscosity can vary widely between manufacturers. You want to search the oil companies websites for Product Data sheets stating viscosity at 40 C & 100 C, and also the viscosity index. A higher viscosity index is good for efficiency, it changes the slope of the viscosity vs. temperature graph, meaning you can have lower viscosity cold while maintaining same viscosity when hot. Mobil 1 0w-40 has a higher index, its an awesome oil for performance vehicles. But in my Corolla I think I'm looking to run thinner weight
Remember, if youre hypermiling, you probably aren't beating your engine where you would definitely want to use a thicker oil.
Mobil 1 0w-20 data sheet:
http://www.mobil.com/USA-English/Lub...l_Economy.aspx
Find the Product Data Sheet link at the bottom of this page:
http://www.mobiloil.com/USA-English/...l_1_0W-20.aspx