I do not think oil is returned to the gas tank it is a waste oil system meaning that the oil is burnt or put out the exhaust. The oiling system is just like the ones on a two stroke glow or gas in that it rides with the fuel all the way to the combustion chamber and out the exhaust. The only difference on the Stihl is that it pulls it in the lower end through a reed valve as the engine piston moves up then the reed valve closes and as the piston moves down it push or compress the fuel oil mix when when the intake valve opens (on a 4 stroke every over revolution) it is forced into the compression chamber. This is done under a positive pressure, where as on most 4 strokes when the piston moves down & the intake valve opens negative pressure or vacuum pulls or sucks in if not turbo or super charged. The ys glow 4 strokes, the stihl and the shindaiwa hybrid engines use this super charging effect by letting the bottom of the piston compress the fuel oil charge before the valve opens.

HTH Hubert