Since then, there have been significant changes in automatic transmission designs and performance requirements, such that today the consumer is confronted with many ATF specifications. Automatic transmissions today may have five, six, seven and even eight speeds. Automatic-manuals, dual-clutch automatics and other sophisticated designs are also on the scene, as are continuously variable transmissions.

Both GM and Ford have introduced their next-generation fluids: GMs new Dexron-VI was introduced in 2005 beginning with its 2006 models, and Fords forward-looking Mercon-LV (which stands for low viscosity) was unveiled in 2007. Ford also has two other licensed ATF products, Ford Mercon V for most pre-2008 models and Ford Mercon SP for vehicles equipped with its six-speed automatics. Official licenses for all prior fluids, such as Dexron-III and plain-old Mercon, have died and cannot be renewed.

Meanwhile, Chrysler licenses products that meet its own trademarked ATF+4 specification, and youll also find several genuine OEM fluids for Japanese and European vehicles. For consumers, choosing the correct fluid is not as simple as 20 years ago.

Of course, it would be desirable to have one fluid for use in all these applications. And indeed some lubricant companies have introduced full-synthetic products that are marketed as multivehicle fluids for use in all these applications. Still other companies market finish fluids that constitute a common base fluid with different top-treat additives, and claim that they meet the performance requirements of genuine fluids.

The problem here is that the specifications for some of these OEM-approved fluids are mutually exclusive, and technically they cannot be met with a single fluid. There are two more problems that result from the use of top treats, as well: (1) Traditionally most top treats are added to base fluids that carry no OEM license, so the addition of an unlicensed top treat to one of these base fluids will still yield an unlicensed ATF, and (2) when top treats are used with licensed base fluids, in most instances the use of these treats will invalidate the license and could impact the transmission warranty, should a problem occur.

In this article, we will discuss some of the product claims being made with respect to universal synthetic fluids and top-treats, and what the consumer should know about these claims.

1. How do the next-generation fluids differ from earlier-generation fluids?

One obvious difference is that the newest automatic transmission fluids (Dexron-VI, Mercon LV and Mercon SP) are lower viscosity fluids than earlier-generation fluids. Their typical kinematic viscosities at 100 degrees C are around 6 centiStoke, whereas older fluids (like Dexron-III, Mercon V and ATF+4) have typical kinematic viscosities in the 7.0 to 7.5 cSt range – that is, heavier. Lower viscosity fluids help provide improved low-temperature performance and less friction for potential fuel economy improvement. Other significant gains with the newest fluids include improved oxidation stability, shear stability and friction durability to help ensure more consistent shift performance over the life of the fluid.

2. Can one fluid meet the Dexron-VI, Mercon V, Mercon SP and ATF+4 specification requirements?

Technically, no, because the kinematic viscosity requirements for these fluids are mutually exclusive in certain instances. At 100 degrees C, the kinematic viscosity requirements for these fluids are as follows:

6.4 cSt maximum for Dexron-VI

6.8 cSt minimum for Mercon V

5.5 to 6.0 cSt for Mercon SP

7.3 to 7.8 cSt for ATF+4

Thus, a Dexron-VI fluid will not meet the viscosity requirements for Mercon V or ATF+4 – and a Mercon V fluid will not meet even the viscosity requirements for Mercon SP. This is true regardless of whether the fluid is formulated with conventional or synthetic base stocks. There also are frictional differences among these fluids that are dependent on total formulation chemistry, not just base stock.

3. What happens if I use a higher viscosity fluid in a transmission designed to operate most efficiently on a lower viscosity fluid?

There is the potential for some loss of transmission efficiency due to slightly less fluid volume being pumped and higher fluid friction. This may or may not affect shift feel, depending on the vehicle and the ambient temperature.

4. Can I use a top-treat to make a higher viscosity fluid such as Mercon V meet the performance requirements of a lower viscosity fluid, such as Mercon LV?

Not very likely. And further, the addition of the top-treat may invalidate the automotive manufacturers license for that transmission fluid. Although you could add a low-viscosity fluidizer to meet the kinematic viscosity requirements, you would still need to meet all the remaining specification requirements, such as shear stability and oxidation stability – without affecting the frictional performance of the fluid.

Most OEMs specifically advise against using top-treats. If the customer experiences a problem with the transmission, and the dealer determines you were using a top-treat or other unlicensed fluid, the dealer could choose to void the warranty on the transmission.