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Differential fluid is the oil that is used to lubricate gear boxes in many automobiles. The differential, or gear box, is the component that is used to translate the rotation of the transmission, usually via a drive shaft, to the drive wheels. A typical differential contains a variety of gears that must remain lubricated during use. If the differential fluid breaks down or leaks out, the gears can overheat, lock up, or even break. There are a variety of different types of differential fluid, and most vehicle manufacturers recommend a specific variety.
When the drive wheels of a vehicle are located on a different end from the transmission, a differential is typically required. Vehicles with rear wheel drive and have the transmission located in the front of the vehicle will usually have a rear differential, while four wheel drive (4WD) and all wheel drive (AWD) vehicles may have both front and rear differentials. Two wheel drive vehicles with transaxles typically do not need differentials, as the transmission drives the wheels directly.
There are three main differential designs that can each require particular kinds of gear lube, though they all perform roughly the same function. Open differentials consist of ring and spider gears, and will typically provide the most power to the wheel with the least resistance. This can lead to a lack of traction if one tire is stuck on ice or in mud. Limited slip differentials are able to direct power to the wheel with more traction, and locking differentials have the ability to provide the same amount of torque to both wheels regardless of traction. Each of these types of gear boxes may require a different type of differential fluid, or specific additives if only a single type of gear oil is available.
Designations for differential fluid may vary between different countries, though in the US the American Petroleum Institute (API) refers to them by gear lubricant (GL) numbers. Most differentials tend to use GL-4 or GL-5 differential fluid. Limited slip differentials typically use GL-5, though they may require specific additives to modify the friction properties of the lubricant. There are also different specifications, such as GL-5+ and GL-5 LS, that are designated for particular types of differentials. Each manufacturer will typically specify whether a certain type of gear lube is required, or if an additive is needed.
Without the correct amount of the right type of differential fluid, the internal components may break down. This may also occur if the fluid is not changed regularly, as contaminants from wearing gears may build up in the fluid, or it may break down from heat. It is often possible to check the quality of differential fluid by removing the fill plug on the side of the gear box. If fluid drips out that usually means it is full, and the fluid can be checked for unpleasant odors or signs of contamination. In other cases the level and condition of the fluid can typically be checked by inserting a wire into the fill hole.
How often should differential fluid be changed? I've never heard the cats down at the local Jiffy Lube make any mention of even checking mine, much less having to change it. In fact, they seem to run the same checks on the rear-wheel drive vehicle that I have now as they did on my front-wheel car I had a couple of years ago.
That seems like something important to find out. I'll ask the Jiffy Lube guys the next time I'm in for an oil change.