Synthetic oil (or cheap "natural" oil) is all very well but you still have to put it through a horrible dirty, unreliable, inefficient, heavy, IC engine. At the moment oil is the obvious thing to use because of the low energy density of alternatives.
If/when energy can be stored in a battery at densities comparable to avgas/jet fuel then electric propulsion has so many advantages it will take over very quickly.
It all depends on battery tech. You can say pah! it will never happen .... but, personally, I would not make too many long term investments in oil (natural or synthetic).