Gear Oils

Out of sight, they should never be out of mind

By Tom Jackson

A lot of attention focused on engine oils in the last few years as environmental regulations and low emissions engines forced nearly a half dozen changes in engine oil formulations.

Gear oils have had it easier, but that’s not to say you can ignore them. While gear oils don’t fight combustion heat, soot, high acid levels and fuel dilution as do engine oils, they nonetheless operate in environments where there is extreme pressure and shock loading. Without the right gear oil you risk scoring, spalling and scuffing of gear teeth in drive axles and premature wear in transmission components. Today’s gear oils have been formulated for better thermal stability, which results in less sludge and varnish, and they also offer greater thermal durability to ensure the gear oil retains its antiwear and extreme pressure properties over the life of the fluid, says Allan Perry, a technical specialist at ConocoPhillips.

Oil companies continue to make small improvements in their formulations, and in today’s cost conscious environment even little improvements can translate into significant savings. Take friction for example. As Dan Arcy, technical marketing manager at Shell Lubricants points out, reducing the fluid friction inside a differential of an on-highway truck can result in up to a 1 percent improvement in fuel economy. Calculate that out to a year’s worth of driving at today’s diesel prices and that’s a savings of around $400.

“That by itself makes it worth your while to replace your old gear lube,” Arcy says. The savings only grow as diesel gets more expensive.

Base oil basics

To get the most from your gear oil selection you need to know how these products work. As with engine oils, gear oils are composed of a base oil and an additive package. Base oils come in Group I, II, III or IV. Groups I and II are the least expensive and comprised of simple mineral oils. Groups III and IV are generally considered synthetic oils and offer superior performance, but at a higher price.

Synthetics are regular mineral oils that go through a process that Mark Betner, heavy duty products manager for Citgo, describes as “extreme polishing and cleaning.” As a result, the molecules in a synthetic base oil are extremely uniform. Betner compares these to a rack of balls on a pool table. When all the balls are the same size you can run your hand over them smoothly. But drop in a few ping pong balls and baseballs and run your hand over it and you have a rougher feel – that’s more like non-synthetic Group 1 and Group 2 oils. These synthetic molecules are called polyalpha olefins (PAOs). They also offer better oxidation resistance at high temperatures and they don’t crystallize as readily either, making them more desirable in cold weather.

Although both synthetic and non-synthetic gear oils are sold today, the majority of gear oils sold are synthetic for the simple reason that the change interval is so much longer for synthetics. Manufacturer recommended gear oil change intervals should always be followed, but it’s not uncommon to see 250,000- to 500,000-mile intervals for on highway trucks using synthetics. For non-synthetics change intervals can run as low as 25,000 to 60,000 miles on highway. For vocational trucks, expect these numbers to be lower.

The only practical reason to use non-synthetic gear oils today, Arcy says, is if your trucks are operating in conditions where contamination is likely, when the differential may be submerged in water or exposed to a lot of mud or dirt. If that’s the case and you know you’ll be changing the oil more frequently than the manufacturer recommended specs, you’ll save money by going with the less expensive mineral-oil based product.


The primary job of additives in gear oils is to protect components against extreme pressure and shock loading. The extreme pressure or “EP” additives most used in gear oils today are sulfur and phosphorous compounds.

The American Petroleum Institute uses a rating system that categorizes gear oils according to how much EP additive they contain. The scale goes from GL-1 to GL-6, although the GL-2, GL-3 and GL-6 categories are no longer valid today. Basically, a GL-1 gear oil is a straight mineral oil with no EP additives or friction modifiers. GL-1 oils are typically used in manual transmissions where stresses are low.

GL-4 and GL-5 gear oils are often called hypoid gear oils. Curved hypoid gear teeth are found in the differentials of all rear-drive vehicles and come together starting on one edge and progressively merging in a sliding action. This requires a wedge of oil to be maintained throughout this sliding action to guard against metal-to-metal contact. And since the pinion on a hypoid gear set does not lie on the same axis as the ring gear, the forces generated are greater than that of gears that line up in the same axis. The further off axis, the greater the force. GL-5 gear oils have the most extreme pressure additives and are spec’ed for hypoid gears with substantial offsets. GL-4 gear oils have about half the EP additive content and are used in applications where the gears are only moderately offset and in some manual transmissions and transaxles.

Shearing happens when the viscosity modifiers in a multi-grade oil break down.

Almost all gear oils also contain additives to resist corrosion and reduce foaming. There are some specialty additives including:

• Pour point depressants and de-emulsifiers. Pour point depressants are used sometimes in extremely cold weather, but most basic 70-weight gear oils will continue to function down to minus 55 degrees Celsius.

• Friction modifiers are also added to gear oils for limited slip differentials to prevent chatter. And in some transmissions special friction modifiers are required to enable smooth shifting.

• Detergents and demulsifiers to neutralize sludge and varnish.

There is also an MT-1 gear oil category. These are intended for non-synchronized manual transmissions in busses and heavy-duty trucks that have seals that may be degraded by GL-4 and GL-5 oils formulations.

Testing prevents problems

Despite the long times between change intervals, you should test a sample of gear oil between change intervals and look for contamination, oxidation, wear metals and shearing.

Shearing happens when the viscosity modifiers in a multi-grade oil break down. Viscosity modifiers are a type of polymer that enhance oil film thickness when temperatures rise. But molecules in viscosity modifiers break apart or “shear” over time leading to a reduced capacity. Some shearing is expected, but by regular sampling you can make sure your gear oil stays within the recommended range and continues to provide the multi-viscosity protection you need.

A one-time sample won’t tell you much about the health of your oil. You need to chart a trend. If you test just once you have no idea of what’s normal, but with regular testing you will establish a baseline for comparison. Then any sudden change in the composition of your oil will alert you to a problem before it turns into a catastrophic failure or expensive repair. EW

Gear oil viscosity

Although the typical gear oil viscosity will be rated at 75W90 or even 80W140, that does not mean gear oils are twice as thick as engine oils. In fact they’re mostly similar in viscosity. The difference comes from the use of two numbering conventions. Engine oils use SAE J306 and gear oils use SAE J300, says John Pettingill, gear oil R&D product specialist at Petro-Canada. So a 90-weight gear oil is roughly the same thickness or viscosity as a 40-weight engine oil.

Additional sources who contributed to this article include:

Victor Kersey, technical director, Valvoline

Richard Gapinski, driveline and ancillary technical service manager,

BP Lubricants USA.

Gary Mondo, product technical support specialist, Chevron Global Lubricants.

*Editor’s note

This is meant to be a general discussion of how gear oils work and what they do and not a recommendation as to what types or what products to use. PTOs, transfer cases and synchronized manual transmissions may have bronze components or other metals that don’t work with some of the additives in some gear oils. And it is possible that one vehicle may require a different type of gear oil in three or four different compartments. So, as with any other lubrication product you choose, do your homework and make sure you’re following every one of the OEM’s specifications to the letter.