Synthetic is the superior lubricate in every way possible for an engine and gear case, besides also increased HP and torque, and yes, reduced fuel consumption by a reduction in friction.
Your talking about synthetic being a superior oil. People talk about this one being better than this one when in fact there's not a whole lot of difference between them. Mobil1, Amsoil, Valvoline SynPower and other synthetic motor oils all have to meet and exceed the same specifications to get that API Logo. The only thing that makes one better than the other are the company's claims and that's all they are is claims.
You Talk about increased HP and torque, and yes, reduced fuel consumption, what kind of increase or reduction are you going to get on any of those when your talking about a lawnmower engine, even with a car there minimal at best. Use The oil you want, synthetic or conventional and change it and the oil filter every 50 hrs and you'll be good to go.