GL-4, GL-5, FD1, TO-4. You’ve probably seen these gear oil specifications in equipment maintenance and lubrication manuals for years, but do you know what they all mean? All of these letters and numbers can get confusing. Is it really necessary to know what each of these specifications means? And can’t you just use any of them in a gear set? They’re all gear oils, right?

Well, yes, they are all gear lubricants, but no, you can’t just use them interchangeably. Axles, differentials, final drives and other gear sets used in mobile equipment applications all require specific lubricants designed for the heavy loads and the sliding and rolling forces associated with the operation of gears.

All gear lubricants are definitely not the same. There are differences in the additive technology used for lubricants for each specification, and just because one type of gear oil works well for one type of gear application, that doesn’t mean it will work well for all.

Different industry associations and different equipment manufacturers issue unique specifications. Most of the time, these specifications are not interchangeable. Substitutions of one type of gear oil for another can sometimes be done, but these substitutions can be tricky and you should use the guidance of a lubrication specialist to help you make the right decision.

For these reasons, it is important for equipment owners, operators and lubrication technicians to identify which specification is recommended by the equipment manufacturer for each machine and each gear set to ensure that the correct lubricant is being used.

The following are some of the most common gear lubricant specifications and their intended applications:

API GL-4 – Intended for gear sets with light to heavy load and sliding forces. GL-4 lubricants are often used in syncromesh manual shift on-road and off-road truck transmissions.

API GL-5 – Intended for heavily loaded gear sets with high sliding forces such as hypoid differentials. GL-5 lubricants contain high levels of extreme-pressure additives to provide protection under boundary lubrication conditions. Common heavy duty applications include differentials in on-road and off-road trucks, differentials, final drives in Caterpillar scrapers, and differentials and planetaries in Euclid haul trucks. Some manual-shift transmissions can use GL-5, but typically should not be used in synchromesh-type transmissions, as the EP additives can be corrosive to yellow metals found in those transmission types. Limited-slip differentials also typically require a specific lubricant with a limited-slip additive.

Cat FD-1 – Intended for final drives and axles in Caterpillar equipment types that do not contain friction materials (i.e. wet brakes). A Cat FD-1 SAE 50 or SAE 60 mineral oil-based product permits a 4,000-hour drain interval and an 80W-140 multigrade synthetic or part-synthetic product permits up to a 6,000-hour drain interval – if the operator is utilizing a used oil analysis program, such as Castrol Labcheck. Common applications include final drives in crawler dozers, as well as differentials, final drives and front wheel bearings in Cat off-road haul trucks.

Cat TO-4 – The SAE 50 and SAE 60 grades can be used in final drives and axles with a 2,000-hour drain interval. Common applications are differentials, final drives and planetaries in Caterpillar wheel loaders, wheel dozers, excavators, and landfill compactors.  TO-4 fluids are also used in transmissions and hydraulics in some machines in lower-viscosity grades.

Universal Tractor Fluids – This category goes by many specifications, such as John Deere J20C, Volvo WB-101, Case (IH) Hy-tran, Ford M2C-134A  (and others), and is intended for axles and differentials in certain equipment that contain wet brakes. These fluids contain friction modifiers to reduce chatter in wet brakes, but should not be used in equipment that is designed for TO-4 fluids, as the friction modifiers that are used may negatively affect braking performance. Universal Tractor Fluids are also used in transmissions and hydraulics in many brands of equipment.

SAE 50 Engine Oil – While not strictly a “gear oil,” this type of lubricant is often the recommended lubricant for some manual-shift truck transmissions.

This article is not intended to provide a complete recommendation of all possible applications for these fluids or for all gear sets. It does provide a quick guide to help clarify the differences in these fluid types and to make users aware that these products are not interchangeable. Please be aware that using the wrong product type can result in poor performance and accelerated wear, as there are no universal fluids that are applicable to all mobile equipment gear sets.

Please be sure to follow the actual OEM recommendation or ask for assistance from your Castrol HD account representative or field engineer before making a gear oil decision.