Automatic transmission fluids (ATF) rank among the most complex fluids used in vehicles. They perform many functions 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.
“Fluid design can play a major role in superior long-term function,” says Steve Rober, national sales manager, Schaeffer Specialized Lubricants. “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 excellent thermal stability to stand up to both high and low temperature extremes,” says Ed Newman, advertising manager, AMSOIL. “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,” he continues. “In the coming years, even smaller transmissions will be responsible for handling as much if not even more torque, so fluids will have even more pressure put on them.”
A Complex Formula
There are numerous varieties and types of ATFs, with differences in viscosity, frictional properties and other factors. Many manufacturers change the formulation of their ATFs, sometimes within a year or two of its introduction. These changes are becoming more common and manufacturers’ recommendations should be taken seriously.
“ATFs can first be generally classified into high and low viscosity oils,” Newman says. “For example, AMSOIL’s 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. “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 ATF’s is their frictional properties,” Newman indicates. “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.”
Automatic transmissions have become increasingly complex, and the need for specific ATF types for specific manufacturers’ transmissions should continue in the future.
“In the coming years, ATFs will have even more demand put on them with new fuel-efficiency CAFE (Corporate Average Fuel Economy) requirements coming into law,” Newman points out. “Systems will need to be even more efficient to meet these stringent new fuel mileage requirements.”
Stick to the Specs
It might be laborious to search through the various ATFs, but it’s essential to always use the correct type. Many manufacturers have their own specifications for the transmissions in their vehicles. Using an ATF not specified by the manufacturer can cause problems such as wear in the transmission and erratic shifting. It can also have a negative impact on fuel efficiency if the viscosity of the oil is not correct.