Increased my mpg by 2mpg on my MazdaSpeed3
 

I switched my engine oil to AMSOIL 5W30 and changed the manual tranny fluid to AMSOIL 75/90 on my 2007 MazdaSpeed3 (now has 5000 miles on it) and I have increased my gas mileage by 2mpg, which isn't too shabby.

I'm driving the same route to/from work, driving the same "style" and all else is the same, so the only thing that changed is the engine oil and gearbox fluid. The fluids are also 100% synthetic and have an extended life versus dino oil.

I'm not affiliated with AMSOIL in any way, just a satisfied customer.

Your car might also be breaking in. But Amsoil (and other good synthetics) certainly help.

I vote for broken in.

Speedy:)

Could be the "break-in", but the mpg increase was noted over the past 200 miles. I'll keep monitoring the mileage (OBD-II scanner) and see if the mileage is consistent, but so far it has been.

Just wait 30 years, it will be leaking all out of the seals and ruin your clutch disk too.;)