Stihl wants you to use 'Stihl Ultra'. The real message, as I understand it, is to use a fully synthetic oil. As others have stated, the valve train is sensitive to pre-mix related carbon buildup.
As an interesting side note, this 'technology' (pre-mix in a 4 stroke) has been used by model engine manufacturers for over a decade. If you're up for some light reading, check out a company called 'YS engines'. YS has developed a method to harness the positive pressure under the piston during the down stroke (typically wasted on 4-stroke engines...vented to atmosphere) to provide positive manifold pressure (boost). It seems that Shindaiwa is attempting something simliar (although they use reed valves instead of the diskvalve in the YS) with their '4-hybrid' engines.
All that being said, the model engines in question are usually run on methanol/nitro based fuels which typically run much cleaner than gasoline.