The SuperTigre G20/2300 was designed with different tolerance and operating pressures in mind, which is why the engine requires a higher-oil fuel. It must have fuel with no less than 18% oil content by volume. The 2300 was designed with FAI Pattern in mind, which means that the fuel draw could be compromised in some of the maneuvers. The other engines were intended more for scale-type flying. When fuel draw cannot be guaranteed to be constant, higher-oil fuel is used to prevent damage from the times when the engine is running leaner due to the different fuel draw rates.
Sound and smoke won't tell you if a fuel is right for the engine, they can only tell you if the engine is getting a good fuel/air mixture. You can get a good fuel/air mixture regardless of the oil content because you have a wide range of adjustment to the needle.