What issue might arise from using gear oil that is too thin?

Using gear oil that is too thin can lead to increased gear noise. Thin oil may not provide adequate film strength and protection between the gears, allowing for more metal-to-metal contact. This can result in the gears producing a greater amount of noise during operation. Additionally, inadequate lubrication can also lead to increased wear on the gear surfaces, which may contribute to longevity issues.

The other potential options relate to issues that might arise from other types or conditions of lubrication. For instance, difficult shifting is typically more associated with thick oil that doesn't flow well in colder temperatures rather than thin oil. Engine overheating usually results from issues within the engine related to coolant or oil capacity, and increased fuel consumption doesn't directly tie to gear oil viscosity but rather the overall efficiency and operating condition of the vehicle. Thus, while thin gear oil can affect overall transmission performance in various ways, the specific issue of more gear noise is particularly notable.