The aviation industry has worked hard to develop strict specifications and quality control measures to the process of supplying a dedicated aviation fuel to ensure it's safe to use for flight operations and reaches the aircraft in good condition. These measures include filtration standards for removing dirt and free water, which could block filters, cause corrosion or impact engine performance. In 2017 an aircraft accident in Ocaña, Spain was reportedly caused by water found in the carburettors, which came from the fuel tank.
While the supply of Avgas is strictly regulated, approval procedures for Mogas are very different and its performance in an aviation application is often untested and unknown.
In addition, Mogas regulations may also allow the use of oxygenated fuel components which can be a potential hazard to aviation. A common oxygenate, ethanol, can cause damage to fuel system hardware and has a low energy content which can reduce aircraft range.
Other factors to consider include the fuel octane quality – if the fuel is of poor combustion quality it may start to explode uncontrollably, which could damage the engine. There’s also fuel volatility and carburettor icing, which can affect engines operating on both Avgas and Mogas, but which are more tightly controlled for Avgas based on aviation experience.
Ultimately it comes down to flight safety and performance. We always recommend you follow the manufacturer’s fuel recommendations for your aircraft. So, if Avgas is your recommended fuel, make sure it’s Avgas in your tank. It will keep your engine flying longer between services.