What is an effect of using inappropriate fuel in an aircraft engine?

Using inappropriate fuel in an aircraft engine can lead to possible overheating of the engine due to the fuel's combustion properties not aligning with the engine's design specifications. Each engine is optimized for a specific type of fuel, which influences the combustion temperature and pressure. When the wrong type of fuel is used, it can burn at different rates or temperatures compared to what the engine is designed to handle. This mismatch can result in elevated engine temperatures, stress on engine components, and potential damage over time.

The other options do not accurately reflect the consequences of using the wrong fuel. Increased fuel efficiency and higher power output are typically associated with the correct fuel type, while the use of improper fuel would likely decrease efficiency and could damage the engine, ultimately not improving its lifespan.