To keep the working parts lubricated, the gearboxes and final drive units have their own oil supply. Most of the time, the oil used is a type known as hypoid oil, and only the recommended grade should be used to top it off. A level check is required, and topping up is probably unnecessary unless there is a leak, which is usually obvious.
When checking transmission oil levels, a dentist’s mirror can be extremely useful. Automobiles with automatic transmissions typically have a lubricant dipstick under the hood, which is frequently checked while the engine is running.