What should be observed regarding hub oil?

Observing hub oil is critical for the safe operation of a vehicle, particularly for buses. The correct choice emphasizes that hub oil should not have leaks and should be at the proper level. Ensuring that there are no leaks is vital as leaks can lead to a loss of oil, potentially causing damage to the wheel bearings and affecting the overall safety and performance of the vehicle. Maintaining the proper oil level is equally important because insufficient oil can lead to overheating and increased wear on components, ultimately compromising the functionality of the hub.

On the other hand, the other choices suggest conditions that would indicate a problem or could lead to failures. Topping off hub oil should only be necessary when the oil level is low, not a constant requirement. Dirty oil can signal contamination and degradation in the lubricant, which can adversely impact its ability to protect components. Allowing for any leaks contradicts the purpose of a proper inspection, as leaks would indicate significant issues that need addressing immediately. Therefore, ensuring that hub oil is free of leaks and at an adequate level is a fundamental part of pre-trip inspections to maintain safety and vehicle reliability.