Gear Lubricant
Of all your automotive fluids, gear lubricant routinely takes the most abuse. Luckily, gear lubricant is a very thick, very strong, and very effective lubricant that is used to maintain everything from your transmission to your rear-end differential. As one can imagine, choosing the right gear lubricant for these automotive parts is very important as neither a transmission nor the differential are inexpensive to replace.
Gear lubricants are very much different from standard automotive oil. Gear lubricants are rated by their “API viscosity,” which very simply means the “thickness” or tensile strength of the lubricant. A higher API viscosity is given to thicker, stronger lubricants and generally suited for use in automotive parts that see the most metal-on-metal interaction.
It is important, too, to understand that 10W40 commonly used as a motor oil in automobiles is not the same as a 10W40 API viscosity. That, however, shouldn't be much a concern as a very simple rating system makes choosing the correct gear lubricant a very simple process.
The 6 classes of gear lubricants from the highest to lowest viscosity ratings are as follows:
API GL-6
The API GL-6 was once a very common gear lubricant, but has lost popularity over time as more people see the GL-5 as the best combination of practicality and high viscosity. This lubricant is most often found in machines, machinery, and “working vehicles” where the machine may assume very large “shock loads” from any number of outside forces. Such force might be a loader or construction vehicle.
API GL-5
The API GL-5 is a very strong oil generally considered to be on par with GL-6 and has since become the lubricant of choice for vehicles or machinery that encounter extreme working conditions. Most commonly, you'll find GL-5 in automotive axles that run either high-speeds or at low speeds but with very large amounts of torque. As you can imagine, greater torque means greater stress on auto parts, and a greater reliance on the lubricant to provide a substantial buffer between moving parts.
API GL-4
It isn't until we get to GL-4 that we reach the level of viscosity that is consistent with passenger vehicles, mostly large buses or transit trucks which serve primarily as a mode of transportation. These are also found on lighter working vehicles such as a tractor where a great amount of torque is delivered to the drivetrain, but is not generally for high speed application. Instead, such torque is usually temporarily employed to navigate hills or minor changes in the slope of the land.
API GL-3
The API GL-3 class is very commonly deployed on what are known as “bevel gears” or those where the gears meet at a 45-degree angle and maintain a circular, or donut shape. The marked difference between GL-4 and GL-3 is the large drop off in the materials used to create an “anti-scuffing” or “anti-wear” level of protection between moving parts. This lubricant often includes as much as 2.7% anti-wear material for “moderate” environments.
API GL-2
API GL-2 is found in autoparts and machinery where a large gear reduction takes place, commonly from a “worm gear.” Worm gears are popular in many different types of transmissions, namely in service work, and require GL-2 as a step up from GL-1 rated lubricants.
API GL-1
Finally, the API GL-1 is a very light, manageable oil designed for applications of very mild conditions of very light machinery and torque. Unlike all other classifications, GL-1 rated lubricants do not always include anti-scuffing, anti-wear additives, or other “friction modifiers.” This is a very light lubricant, and is very much intended only for very light usage.
Getting the Right Lubricant
Automotive and machinery manufacturers do a very good job of indicating the lubricants to be used, in what quantity, and how often they are to be renewed or replaced. Gear lubricants are some of the least expensive, but the most important, of machinery maintenance and it is not recommended, under any circumstances, to delay or disregard the replacement of lubricants in the most important parts of a drivetrain.
