Four-wheel and all-wheel drive vehicles have an additional gearbox known as a transfer case. This sends the drive power from the engine to the front and rear axles. As traction conditions change, the transfer case helps to shift power from one axle to another. Lubricating fluid in the transfer case keep the gears cool and running smoothly. Replacing this fluid at the right mileage is imperative to help prevent premature wear and damage that gets caused by contaminated or broken-down lubricants.