mobil 1

I was keep reading on forum that 5w30 is better oil compare to 0w20 factory recommended. I took the chance got Mobil 1 Extended Performance 5w30 and fuel average dropped to 12.5mpg compared to Motul 8100 0W-20 Eco-lite gen3 which was given me 20.0 plus mpg. car is full stock, any idea what it...