[beaker]

I am certain that there is a fuel economy decrease using 5W-40. That said, I am also certain that there would be no way I could measure the difference using scientifically valid standard deviation on a single vehicle within normal timeframes. Be it the frequency of regeneration, vehicle speed, headwind, tailwind, winter fuel blend, summer fuel blend, the variables would simply be too great. That said, my 3.0L Volkswagen ran factory-approved oil while under warranty (either 0W-30 or 5W-30), which I immediately swapped to 5W-40 Rotella when the factory powertrain warrany expired at 120,000 miles. I have only added 16,000 miles to that car since then and cannot discern a difference in fuel economy. If one exists, I think it is on the order of 0.5 mpg (2% - 3%). I can probably give you a more definitive answer as I approach 200,000 miles on that car.