First things first: Rear axles have gone through some changes from the beginnings of the automobile. Originally, there were bevel gears that later morphed into spiral bevel gears and worm gears, and ultimately became hypoid gears. I wont bore you with the details, but the primary drivers for this evolution were noise reduction, increased load-carrying capability and, as a bonus, lower vehicle profiles.

By the 1960s, rear axles with hypoid gear sets were found in nearly all the vehicles on the road. In addition, commercial heavy-duty trucks, light trucks and some muscle cars were using either regular rear-axles or limited-slip axles with a design that allowed the power from the engine to be directed to the wheel with the most traction.

Lubricant requirements for these axles were originally set in the early 1940s by a military universal gear oil specification, MIL-2-105. Before this, as former U.S. Army fuels and lubricants expert Maurice LePera has noted, there was really nothing to define lubricants. But the Army, going into World War II, urgently needed to assure satisfactory performance of lubricants for engines and axles.

From then on, there was a progression of specification updates, ultimately leading to MIL-PRF-2105E. This was superseded in 1998 by the global standard SAE J2360, Automotive Gear Lubricant for Commercial and Military Use. SAE J2360 is updated and reissued regularly, most recently in April; for a qualified product list see www.prinetwork.org.

Among the changes and upgrades over the years have been tests to measure gear oil performance regarding oxidation, water corrosion, heavy loading and shock loading. In addition, there are copper corrosion and antifoam requirements.

Meanwhile, in 1969 the American Petroleum Institute introduced the GL classification system, officially API 1560, Lubricant Service Designations for Automotive Manual Transmissions, Manual Transaxles, and Axles. It lists numerous classes, although many are obsolete. The active ones are GL-4, GL-5 and MT-1, with MT standing for manual transmission including those for heavy-duty trucks.

GL-4 is a special case because the tests that were developed to measure the performance parameters of this category are no longer available. However, there is a thriving business in selling GL-4 gear lubricants, so an accommodation was made by API to continue these products.

Although this service designation is still used commercially to describe lubricants, test equipment for performance verification is not currently available, advises the 2012 edition of API 1560. It goes on to say, Lubricant end users are advised to request appropriate supporting documentation on previously tested lubricants from their suppliers.

That caveat seems innocuous enough but it has generated a great deal of concern and comment over the last year or two. Under standard API practice, GL4 should be inactive. There are no tests to measure performance or gain acceptance for new products, which is usually the death knell for API categories. However, GL-4 has a solid place in oil marketers product lines and customers for whom it is just the right thing. Not large but niche filling, and those using it dont want to change. So GL-4 abides.

The U.S. light-duty marketplace is not a growth area for gear lubes, as front-wheel-drive transaxles have taken over most of the passenger car market. These components are lubricated with ATF, not gear lubricants. A few light-duty vehicles still have rear axles, and for these the standard gear lube is one meeting the API GL-5 category. (My truck is one of those dinosaurs, and uses an SAE 75W-90 synthetic gear oil to extract as much fuel economy as possible.) Ian Macpherson, marketing director for commercial and industrial at Afton Chemical, pegs the U.S. market for on-road gear oils at roughly 50 million gallons a year. The most widely used specification is SAE J2360, he told LubesnGreases, though there is a lot of plain old API GL-5 out there. Extended drain products are also doing well with specific OEM qualifications.

Whats new with these lubricants is the same as with engine oils: better efficiency to improve fuel economy. As I noted, my 2001 GMC uses SAE 75W-90 synthetic gear oil, GM Spec 9986115. This low-vis oil results in lower friction and less churning in the gear case; both help improve fuel economy. Most OEMs have similar specifications for gear lubes in light-duty vehicles.

On the heavy-duty side, the story is similar – except gear lubes are still a major part of the market. Truck gear oils today include lower-viscosity and multi-viscosity products that try to capture every drop of fuel efficiency. Most axle manufacturers, and some OEMs, have their own specifications as well, which usually are very similar to the J2360 standard. Builders Mack, Meritor and Eaton all call for synthetic gear oils in SAE 75W-90 and SAE 75W-140 viscosity grades. For example, Eaton requires approved synthetic lubricants be used in its Fuller transmissions and Dana axles in order to qualify for extended warranty coverage and up to 500,000-mile initial drain intervals.

The additive technology behind gear lubricants is pretty straightforward. The primary components are anti-seize additives such as sulfurized and phosphorus-containing compounds; friction modifiers; rust inhibitors, and oxidation inhibitors. In addition, there are antifoam agents (bubbles dont lubricate), pour-point depressants and increasingly, viscosity index improvers.

Gear lubes also use all of the API base stock Groups, and increasingly use synthetics (i.e., hydrocracked Group IIIs and Group IV polyalphaolefins, plus some Group V esters to improve additive solubility). This shift to synthetics is due to OEMs moving to lighter vis grades such as SAE 75W-90. SAE 75W-90 is now the dominant viscosity, confirms Aftons Macpherson. Larger fleets use less and less 80W-90 anymore.

Current plans for gear oils dont seem to include any new or exotic viscosity grades (like the SAE 16 being developed for engine oils). Synthetic SAE 75W-90 or SAE 75W-85 are about it for now. However, dont be surprised if some OEMs start to look at lower-vis grades in the near future. With new federal mandates set to come into force in 2016, every bit of fuel economy gain is important.

Is there a future for automotive gear lubricants? The answer seems to be a qualified yes. So long as there are rear-wheel drive vehicles, there will be some sort of gearing system and a need for gear lubricants – and better ones. Rear-wheel drive may be in decline – but it isnt dead yet!