API GL-04 vs API GL-05
Gear oil is a very important component in most machinery and automobile transmissions as it keeps the friction between parts minimal and reduces the wear and tear. Different types of gear oil are classified by the American Petroleum Institute, which is more commonly known as API. The two most common types of gear oil that are used in automobiles are the GL-04 and GL-05. The main difference between GL-04 and GL-05 oils is the conditions that they are meant for. GL-04 is meant for light to heavy conditions, typically ranging from small cars to buses and tractors. In contrast, GL-05 oils are meant for severe conditions where GL-04 may not provide sufficient lubrication; specifically, for hyphoid gears.
The GL-05 contains more than 50% more scuffing additives than GL-04. Although this contributes to the performance of the oil, it can be problematic in some cases as some types of GL-05 oils contain aggressive chemicals that may eat through yellow metals like bronze or brass. Most gearboxes use yellow metals, which is why GL-04 is strongly advised to prevent damage to the components inside the gearbox. There are some GL-05 oils that do not use these aggressive chemicals and are safe to use in gearboxes, but you need to make sure that this is the oil you have before putting it in.
Aside from the problematic gearbox, GL-05 oils are advised for most other applications. Its performance is easily evident in how hyphoid gears benefit from it. Hyphoid gears experience a much greater amount of pressure due to the wiping motion involved in the contact. The extreme pressure additives in GL-05 oils help the hyphoid bevel gears to cope with the much greater pressures.
The general rule is to use GL-04 oil in your gearbox and GL-05 on the rest. But to be on the safe side, you need to consult the owner’s manual of your vehicle to see on what are the recommended oils to be used.
Summary:
GL-04 is intended for light to heavy conditions while GL-05 is meant for severe conditions
GL-05 contains more anti-scuffing additives than GL-04
GL-05 maybe corrosive to some metal types while GL-04 isn’t
GL-04 is advised for synchronized gearboxes rather than GL-05
GL-05 is used for everything else aside from gear boxes
GL-05 is better for hyphoid gears than GL-04