One of the reasons for the proliferation of specifications is the wide number of transmission designs out there. Another is the fact that there is no industrywide set of transmission fluid standards, as there is for engine oils. Id like to look at both issues and how they affect the transmission fluid market.

First, lets address the issue of transmission designs, starting with a personal observation. Recently, my wife and I took a two-week vacation to Italy. Most of the time we were aboard a wonderful cruise ship sailing around the Italian peninsula, but while in Rome we rode in cars and vans which shuttled us from the airport to our hotel, to the ship, and finally back again to the airport. Thats not to brag but as a way of introducing the subject of Mercedes-Benz commercial vehicles, specifically those equipped with diesel engines and dual-clutch transmissions (DCT). These were first used in passenger cars in 2003.

In our Roman van, the DCT featured two wet (in oil) multi-plate clutches, which allow gears to be engaged on two separate shafts at the same time. The clutches are activated electro-mechanically, engaging and disengaging in unison, routing power from one gear set to another without loss of drive. The result: smooth gear changes and improved fuel economy.

Our driver was very complimentary about her van during our one-hour trip around Rome. She felt it had tremendous acceleration and shifted effortlessly. I can testify to the fact that the van did accelerate well and seemed to shift without any sensation of shifting.

According to consultant Roy Fewkes, who formerly was a transmission engineer at General Motors, many wet DCTs have two separate oil reservoirs. One holds the fluid that acts as a hydraulic oil and manages the shifting mechanisms, among other activities; the second contains a gear oil to actually lubricate the gear boxes themselves. (In some designs, the fill caps for both oils are adjacent to each other and are not clearly identified as to which oil goes into each. One shudders to think what might happen.)

In any DCT design, the challenge is to provide sophisticated frictional properties for the clutch; superior protection for the gear sets; and state-of-the-art synchronizer performance across a wide range of synchronizer materials.

As Fewkes reminds us, there are many other transmission designs out there, such as continuous variable transmissions which are favored by Asian automakers. These, along with DCTs, are pushing into the North American market. All these new designs call for distinct fluid technologies, but one thing they have in common with step-transmissions is that the OEMs are demanding longer fluid drain intervals and, in some cases, fill-for-life capability.

As well, automatic step-transmissions are changing in design from five-speed to six and now even seven-speed capability, improving drivability and performance. Meanwhile, to improve fuel economy, transmission components are getting lighter despite increased engine horsepower. And new fuel-efficient transmissions exhibit higher torque and higher operating temperatures than before.

As an example, back here at home in Arizona, my 2008 Nissan Quest has a five-speed transmission which is a remarkable piece of hardware. We drive on the most difficult roads and feel that we have exceptional control of the vehicles speed and handling as well as getting excellent on-highway fuel economy. It works hard, and so does its transmission fluid.

Getting back to the original question: Why are there so many ATF specifications in the marketplace? One answer is that different companies have their own unique ideas about how a transmission should behave. The classic divergence was Fords ATF Type F fluid vs. General Motors ATF Type A, Suffix A (both now obsolete). Ford required Type F for a number of years for one basic reason: Ford engineers liked to feel the shift. Type F didnt contain friction modifier, so feel it they did.

On the other hand, GM wanted a smooth, relatively undetectable shift so its ATF Type A, Suffix A did contain friction modifier.

Later, Ford moved away from the hard shift of Type F to the more pleasing smooth-shift and introduced its proprietary Mercon specification. GM had already updated its proprietary transmission fluid requirements to the Dexron series. For a while both fluids performed in similar fashion and oil marketers could get combined approvals; one fluid could satisfy both. That didnt last though, and the two separated again around 2005.

In the meantime, Asian and European OEMs added to the fun with their own requirements. These too were often contradictory to each other, mainly due to differences in frictional material properties or different ideas about durability vs. performance. Seal materials also differed, creating another headache. Trying to tie all this together is like a juggler who keeps adding more and more balls into the act.

ATF is probably the most complex of all lubricating fluids. Not only does it have to reduce friction to prevent wear, like all lubricants, but it also has to allow a certain level of friction so clutch materials can engage. Since most OEMs use proprietary frictional materials, virtually every ATF is OEM-specific; in some cases, theyre transmission-specific.

In addition, ATFs must be compatible with specific transmission components, operate at both low and high temperature extremes, and maintain constant performance for extended periods – sometimes for the life of the component. No problem, right?

As you can see, getting the American Petroleum Institute or other governing body to set industrywide standards for ATF would be pretty difficult, due to the competing OEM requirements. Back in 1937, when the SAE paper was presented, the issue was relatively simple. There were no automatic transmissions so the manual gearbox was the only game in town. Yet there was confusion about what type of chemistry and what viscosity would be needed. Even then, the OEMs were each going its own way.

Today, ATF typically contains the components shown in the table below, which may seem familiar to those who blend engine oil. The chemical make-up of each ATF is unique though, and wholly different properties are emphasized versus what youll see in engine oils or even gear oils. And getting the precise blend of additives and base oils is difficult.

As you can see, the balance of ingredients is crucial to the transmission for which the lubricant is designed. Thats why there are so many products out there. Each OEM has unique combinations of clutch plate and bands. There is also the matter of the choice of seals. If seal elastomers were standardized, it might be easier to reduce the number of formulations. But thats a competitive issue for the OEMs, not something the oil companies can control.

Base stocks are another issue. Many oil formulators are now using API Group II and even Group III base oils in their most current fluids, in order to reduce oil volatility and improve oxidation resistance. In the future, there is likely to be a move on the part of some OEMs to include more Group IV and V fluids to gain even more stability, as long drains and seal-for-life become the norm. If the transmission cant be opened or the fluid topped up, its one less headache for them.

Last April, Brad Onofrio of Afton Chemical wrote an excellent article for LubesnGreases on the subject of multi-vehicle ATF. In his article, Onofrio noted that the North American ATF market is primarily Dexron/Mercon (52 percent) with the rest divided among numerous other OEM specifications. This despite the fact that both Ford and GM have spent nearly 10 years trying to get consumers to adopt newer fluids, and neither supports the old Dex/Merc specifications any more.

Onofrio warned that multi-vehicle products (as well as aftermarket additives) are a risky bet when dealing with a transmission that is under warranty. His bottom line advice to such drivers is to follow their owners manual recommendations.

Whatever choices the OEMs make, the oil and additive industries are in the game to make it work. Whether its new components or new oil formulations, there will always be a need to improve. It should be very challenging for the industry.

Industry consultant Steve Swedberg has over 40 years experience in lubricants, most notably with Pennzoil and Chevron Oronite. He is a longtime member of the American Chemical Society and SAE International, where he was chairman of Technical Committee 1 on automotive engine oils. He can be reached at steveswedberg@cox.net.