Motor Oil Improves Milage?

I put a quart of valvoline motor oil in this morning and read on the package 'improves gas milage' or something along those lines. I don't see how oil affects that sort of thing, but I'm thinking this claim doesn't have much research to back it up.

Also, I think Valvoline has motor oil out for high milage engines (75000+). I may look into that the next time I'm a bit low on oil inbetween changes. Not sure if it'll really make a difference or not, but I dont have a problem giving it a shot.