CHOOSING ATF
Different ATF's (Automatic Transmission Fluid) can have different frictional properties which can produce different shift characteristics. You may have already experienced problems like lock-up shudder, or squawks on shifts and have corrected them by changing the fluid. This alone tells, you that friction material and fluids are critical in today's cars.
MEETING THE O.E.M. SPECIFICATIONS
The first thing to consider when choosing an ATF is "Does it meet the O.P.C. specification?" ATF's wishing to be labelled as DEXRON II and/or MERCON must first meet the respective minimum requirements. It's important to note that even though the specification for DEXRON II and MERCON are currently very similar, THEY ARE NOT IDENTICAL. Also note, even fluids which meet the same specification may not be identical. One fluid may just meet a specification and, another may far surpass it. You should know what your fluid properties are ! You can get that information from your fluid supplier.
EVALUATING YOUR FLUID
Ask your supplier to certify that the fluid meets O.P.C. specifications (MERCON OR DEXRON II). He will do that by supplying you with the license number issued to him by the O.P.C. A DEXRON-II license number (sometimes referred to as a "D" number) will always start D-2. MERCON license numbers will be six digits starting with M.
TRY TO MEET SEVERAL SPECIFICATIONS !
DEXRON II and MERCON have different minimum specifications, so a product that meets BOTH specifications may be better then those meeting only one spec. Meeting other specs, in addition to the first two can be an added benefit. If a fluid is licensed as DEXRON II AND MERCON as well as others like Allison C-4, or Caterpillar TO-2/TO-4, it means the fluid had to pass more tests and may be a better fluid.
Lastly, demand that the license numbers be placed on your invoices. If your supplier is unwilling, it is very likely that they are supplying you an unlicensed fluid. Licensed suppliers are required to supply the license numbers to their customers as part of their agreement with the O.P.C
RECAP
Only use fluids with O.P.C. approvals.
Try to get a fluid that meets more than one spec (e.g DEXRON II AND MERCON)
Check the viscosity at -40 degrees. The lower the number the better.
Avoid "cheapo" fluids with no license numbers.