Just thought I'd post on this since I keep reading others results..... I've been keeping miles per gallon tabs on my 2000 Honda Accord V6 since I bought it new. A couple of weeks ago I changed the oil in it to the new Mobil-1 5w40 Truck/SUV oil. My first test of MPG was 25.5 which is just about the same average since it was new and since I was using the 0w30 weight mobil-1. I had similar results when I switched my 2003 Honda CRV from the 5w20 to the new Mobil-1 0w30 regular oil and now the new racing oil. No detectible difference. My conclusion is oil vis. has little noticible effect on gas mileage at least in my two cars. Possibly biger or more extreme jumps between vis. may make more difference.