Motor gasoline is manufactured to much looser specifications than that of aviation gasoline. Quality and performance vary widely from refiner to refiner and from location to location. Quality control and quality ensurance in motor gasoline is much less stringent. The risk of contamination is also greater due to less careful handling. Also, many components of motor gasoline, especially detergents and oxygenated fuels, are quite variable in type and proportion and are generally not known or readily detectable.
Motor gasoline has a much wider distillation pattern than avgas.
This can result in poor fuel distribution, poor anti-knock component distribution, and excessive motor oil dilution. Motor gasoline is generally more volatile than avgas and could lead to increased vapor-off, vapor lock and carburetor icing. The anti-knock properties of motor gasoline are also different. While the octane ratings appear similar in number, the tests are conducted differently and are not comparable. The stability of motor gasoline is also much lower than avgas.