What should be checked to determine the oil level?

The correct choice focuses on using the dip stick to determine the oil level. This method is standard practice for checking engine oil in vehicles, including those requiring a Class A CDL. The dip stick is a metal or plastic rod that provides a clear indication of the oil level within the engine.

When you pull out the dip stick, you can wipe it clean, reinsert it, and then remove it again to observe the oil level on the stick. There are usually markings on the dip stick that indicate the correct operating oil level, allowing for an accurate assessment of whether the oil is at an adequate level.

While manual inspections could involve checking for leaks or examining other aspects of the engine, they do not provide the precise measurement required to determine the oil level. Visual inspection alone may not give an accurate reading of the oil, and assessing engine performance is not indicative of oil level, as performance issues may be due to various factors unrelated to the oil level itself. Therefore, using the dip stick is the most reliable and recommended method for checking oil level in vehicle maintenance.