Final answer:
Metal gears must be lubricated with gear oil to reduce friction and wear between the gear teeth.
Step-by-step explanation:
Metal gears must be lubricated
, which is accomplished with gear oil. Lubrication is necessary to reduce friction and wear between the gear teeth, allowing them to mesh smoothly and transfer power efficiently. Gear oil forms a thin film between the gear teeth, providing a protective barrier and enhancing the lifespan and performance of the gears.