The reduction of friction in the engine will make it more energy efficient. Two miles per gallon increase is approx. a 4-5% increase. That's a pretty good improvement. It may also be due to the cooler temps. Colder air is denser and therefore has more "energy" in it. It allows for a denser charge of air into the air/fuel mix. Keep us posted if your mileage maintains this improvement when the weather gets hotter.