Odd that this should come up again just now. I've seen some increase recently, with a 4 oz./10 gal. dose (7 oz. in my 18-gallon tank). I gassed up at the cheapest non-SinoMart station near me the Sunday before Mardi Gras. On Fat Tuesday I took a little run out Highway 90, flat smooth 55-mph cruising, about 40 miles round trip, which probably helped. However . . .
My usual gas mileage for the last couple of months has been about 20.8 over 10-12 days, about 70% city. During this last tankful the gas gauge, that lying robot, dropped more slowly. On my fillup on Friday, I clocked 22.8!
Yes, the open-road cruise was probably most of it -- but maybe MMO helped a bit. Currently I'm testing, keeping the variables as much the same as I can (same gas station, same pump, same early-morning cruise once during the tankful), but without MMO.