AUTOMOTIVE GEAR OIL: WHAT’S THE DIFFERENCE?

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?Automotive gear oil

Automotive gear systems, such as those in manual transmissions and differentials, have a tendency to engage in a lot of metal-on-metal contact, causing heat and friction which results in wear and potential component failure if not maintained properly. Using the right gear oil for each application helps protect these moving parts by reducing friction and transferring heat away from them.

What is a gear?

An automotive gear is a rotating toothed cylinder, which forms a mesh with another toothed machine part or gear and transmits torque. The gears can change torque, speed, and direction of the power source and are a critical component in an automotive drivetrain. Gears come in many designs, such as spur, helical, bevel, worm and hypoid designs.

?Gear oil vs engine oil?

Gear oil is a lubricant used specifically for automotive manual transmissions, differentials, transaxles, and transfer cases. It helps the drivetrain run smoothly and protects critical internal parts in a vehicle’s gear systems from wear and heat damage. Inadequate lubrication can result in scouring, corrosion, scuffing, and pitting of important drivetrain components. This deterioration can compromise a vehicle’s performance, and eventually lead to costly repairs.

Gear oil has a higher viscosity than engine oil. It is identified by its performance classification and its viscosity. The Society of Automotive Engineers (SAE) publishes a viscosity grading guide (J306) for gear oil, which is a different grading scale than the SAE’s engine oil scale (J300).

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Gear oil API performance classifications

There are several automotive gear oil designations, but the two most common gear oil designations – Lubricant Service Designations for Automotive Manual Transmissions, Manual Transaxles, and Axles – are set forth by the American Petroleum Institute (API) for passenger vehicles:

API GL-4 classifies passenger car and light truck manual transmission lubricants and oils used in tractor gearboxes and spiral bevel gear final drives.

API GL-5 defines passenger car, light, and heavy truck and bus hypoid final drives where there is a large amount of sliding movement across the gears.

A number of automotive manufacturers require lubricants that meet their own in-house specifications. Mercedes-Benz, GM, Ford, Volvo, and others have specifications for the gear lubricants to be used in their equipment, particularly where special warranty or extended lubricant service is offered.

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Gear oil viscosity

There are two common passenger vehicle gear oil viscosities.

75W-90

The 75W-90 gear oil is used under extreme pressure conditions and can be recommended for limited-slip differentials and for use in colder climates. It is commonly used rear differentials, some final drives, and axles. The thick oil film formed this oil can withstand high temperatures.

80W-90

The 80W-90 gear oil can be used in extreme pressure conditions and as a lubricant for some non-synchronized manual transmissions in heavy-duty trucks or buses.

Monograde Gear Oil

The SAE classifies gear oils suitable for operation at one temperature on a monograde scale. Gear oil meant for high ambient temperatures is designated by a single number, such as SAE 50. Higher numbers indicate a more viscous oil.

Multigrade Gear Oil

Multigrade gear oils have additives that change their viscosity at different operating temperatures. These gear oils are labeled with a rating for the oil’s low- and high-temperature viscosity. For example, SAE 80W-90 gear oil has a low-temperature rating of 80 and a high-temperature rating of 90.

Types of gear oils

Rust-Prevention Gear Oils

Rust-preventing gear oils are petroleum-based or mineral-based and contain rust inhibitors. These gear oils come in two different classes: monograde and multigrade. Monograde gear oil is designed for use in “normal” temperatures while multigrade gear oil can be used at hotter and cooler temperatures. Rust-prevention gear oils protect and extend the life of gears running at higher temperatures and are best used in low-load gearboxes.

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Extreme-Pressure Gear Oils

Extreme-pressure (EP) gear oils can be synthetic- or mineral-based. EP gear oils are commonly multigrade and contain extreme pressure, rust inhibitors and anti-foaming additives. These additives allow EP gears oil to protect highly loaded gear sets. High contact pressure of the gear tooth face can cause gears to wear quicker. EP gear oils help extend the life of a high load-bearing gear sets.

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?Synthetic Gear Oils

Synthetic gear oils are frequently multigrade oils that are formulated robust additive chemistries. Thes gear oils can be used at extremely high or low temperatures. They prevents rust, foaming of the gear oil inside the gearbox, and wear of the gears. Synthetic oils are used in gearboxes that are run in severe conditions such as sub-zero environments, extreme heat, wet climates, and dusty or dirty conditions.