Why selection of the appropriate ATF is essential

Automatic transmission fluids (ATF) are complex lubricants that perform a variety of functions in today's engines.


Automatic transmission fluids (ATF) are some of the most complex fluids used in motor vehicles. They perform many functions in an engine including lubrication, heat dissipation, wear protection, shift quality and more, and are optimized for the special requirements of a transmission, such as valve operation, brake band friction and the torque converter, as well as gear lubrication. ATFs are even used as a hydraulic fluid in some power-assisted steering systems, as a lubricant in some 4WD transfer cases, and even in some modern manual transmissions.

“ATF’s lubricate the most complex and sensitive major component of today’s vehicles,” says Steve Rober, National Sales Manager, Schaeffer Specialized Lubricants, a manufacturer of specialized motor oils, fuel additives, synthetic engine oils, industrial lubricants; www.schaefferoil.com. “Fluid design can play a major role in superior long-term function. Complex components, such as automatic transmissions, must have lubrication that cools, lubricates, resists oxidation from the high temperatures, and allows clutch plates to smoothly engage and disengage thousands of times.

“As we look forward, we can only see more complicated multi-speed and electronically-controlled units requiring better fluids.”

“Transmission fluids need to have excellent thermal stability to stand up to both high and low temperature extremes,” says Ed Newman, Advertising Manager, AMSOIL, a manufacturer of synthetic lubricants and filters. www.amsoil.com. “In high temperatures, transmission fluid can’t get too thin or it won’t protect the vital gears and components of the transmission. The fluid also needs to prevent varnish and sludge deposits that can clog narrow oil passages.

“Transmission fluid has to be able to flow in extremely cold conditions to prevent slow shifting, which leads to a loss of fuel economy, and needs to provide excellent wear protection and frictional durability,” Newman continues. “In the coming years, even smaller transmissions will be responsible for handling as much as, if not even more torque, so fluids will have even more pressure put on them.”

Numerous varieties

The varieties and types of ATFs are substantial. Differences in viscosity, frictional properties and other factors create the necessity for more ATF selections. Many manufacturers have changed the formulation of their ATFs, sometimes within a year or two of the ATFs’ introduction. These changes are becoming much more common and manufacturers’ recommendations should be taken seriously.

“ATFs can first be generally classified into high- and low-viscosity oils,” AMSOIL’s Newman says. “For example, Dexron III was a standard General Motors transmission fluid for many years until it was replaced by the lower viscosity Dexron VI product. The major reason for lowering the viscosity is to improve fuel economy. Many OEMs now offer or require lower viscosity transmission fluids for their vehicles.

“The best description of fluid types is to describe them as conventional or mineral oil-based lubricants, synthetics processed from conventional base oils and synthesized hydrocarbon synthetic type fluids,” says Mark Betner, Mystik Product Manager, CITGO, a refiner, transporter and marketer of transportation fuels, lubricants, petrochemicals and other industrial products; www.citgo.com. “Other lubricant descriptions and types may be rated using a manufacturers’ performance rating or the American Petroleum Institute (API) standard, such as GL 1 through GL 5.”

“Another distinct difference between ATFs is their frictional properties,” says Newman. “Each transmission design may use different friction materials and the fluid has to be designed to work with these materials to avoid excessive slipping and or chatter.”

Newman adds that automatic transmissions have become increasingly complex, and that the need for specific ATF types for specific manufacturers’ transmissions should continue in the future.

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