Question about Miles Per Gallon.?

Ok a few weeks ago I changed my cars oil. I did it quickly before work and accidentally used 5w20 instead of 5w30 (Mobil 1) in my camry (5-w20 is for my van). After I came back home I realized my mistake and contacted Mobil to see if it would be a problem. They responded with telling everything will be fine just keep your current oil change interval and change back when your interval comes. Now everything is fine and dandy except for one thing - I now get around a extra 20-30 miles per tank. My driving habit, route, octane (87) has stayed constant. The only diffidence that I can see is the oil. How does using 5w20 increase gas mileage. Obviously I'll change back pretty soon (Next OCI). I just thought it was really strange.