I race 4 stroke engine go kart, and a very respectful engine tuner said to me that if I run with little less oil then the engine level (80% of recommended, 0,8l) it will break sooner but will become more powerful. The engine is a honda gx390 tuned. I did some research but found nothing. Somebody has a explanation or even a tip? thx
The only theory that seems plausible is to reduce rotational resistance of the crank moving through the oil. This could in theory allow the engine to reach maximum rpm sooner. These engines typically use a dip or splash lubrication method. The spinning crank dips into the sump and distributes the oil. There is no pump to circulate the oil. As your questions state you are sacrificing durability for performance, not really an uncommon feature in performance engines.
The oil specified in the sump is to make sure that the oil pump does not suck air when in angled situations - the minimum mark is usually sufficient unless you are really pushing the angles.
The other function of the oil is to remove heat, and dirt obviously, and that is the other basic reason to have the level correct.
I have not heard of running lower than the minimum for power, but most racers choose an oil that performs well at high engine speed and is not too viscous etc to minimise the pumping losses while providing excellent engine protection.