None of us drives in a laboratory so we have to be concerned about the dozens of other variables, almost all of them more important to mpg than a 30 to 40 oil grade change:
Ambient temperature, wind direction, altitude, road conditions (type of surface, texture, cracks, wet/dry/icy, etc.), tire type/size/pressure/age, type of driving you do (speed, lead foot, short or highway trips, braking style, hilly or flat terrains, excess idling, drafting), vehicle type (size, weight, age, tune/efficiency, mfg tolerances, windows open/shut, AC use, engine displacement, drive train gear ratio and how you employ that), type of fuel you use, how quickly you warm up your engine, maintenance, etc. No doubt I've missed some, but anything but oil grade is part of this list...making up 99% of mpg factors. Many of these factors change by the minute. Some of them you can control by where, when and how you drive.
So once we've optimized all of the above in our personal vehicles, and making the best choices on how, where, when we drive...even what vehicle we buy, then we can be concerned about that last 1% of potential fuel efficiency by adjusting oil grade. That excess 40 lbs of stuff you have sitting around in the car? That's probably a bigger factor than oil grade.
Then there's the issue of what oil grade actually maximizes the life of your engine. It might not be the factory recommended grade, particularly if it was one that was back specced. Your mpg savings of $10-$50 per year might not balance out to the cost of a new engine 8-20 years down the road if you are one of those who are keeping your car until it's just no longer feasible. If you lease, or only own a car to just past the warranty period, then by all means use the thinnest oil recommended to maximize the mpg and cost of operation. You don't care about what happens to the car after you sell it.