If you beleive hard enough and try to justify then maybe it will happen.
Pennzoil conventional normally is one of the best in many vehicles for MPG. (all within 1mpg.. Syn vs conventional vs brands)
Worst MPG I've had by about 3/4 mpg was old Mobil Drive Clean quite a few years ago.
This data is over 5k miles OCIs (or more) and on paper. Not trip computers in dash. GPS verified speedo/odo and filling up at the same pumps.
Take care, Bill
I'm running Long Life Gold 15W-40, which has more anti-wear agents than friction modifiers based on what I have read, and I still get the same mileage. That said, I use a maintenance dose of MMO in the fuel tank and there's a decrease in mileage only if I don't use it. Did you check the basics - air filter, tire pressures, etc?