A UOA on trans fluid is only useful if you have a particle count done and get an ISO cleanliness code for the oil. Also, you want a TAN test (read on)
You want an ISO cleanliness code of 19/16 or lower (according to Eleftherakis and Khalil, two engineers that "wrote the book" on trans fluid contamination). That's the generic upper "normal" limit... lower is better. Individual manufacturers may have their own limits but you'll play Hobb finding what they are... and they may vary from trans to trans within an OEM family.
Often, the metals build up to a high & dangerous level before the fluid is worn out because the filtration in an automatic is minimal. The spectral analysis in a normal UOA will only "see" particles smaller than around 5 microns and particles in the 5-15 micron range make up 82 percent of the total on average, so you are only seeing the contamination from that 18 percent of smaller particles. Most of the experts I've consulted with (Including Abe Khalil) think that you can't read the "tea leaves" very accurately from a normal UOA.
Also note that an average automatic generates 75 percent of it's lifetime of contaminants in the first 5,000 miles of service, so the first fluid change is the most important one. That comes from the stuff "built-in" from manufacturing and later from break-in. After break-in, the amounts level off pretty well. Also note that the Japanese, in general, were at the front of the industry effort to clean up trans assembly procedures to eliminate the built-in crud... and deliver fewer warranty-stricken units. The past few years have brought big strides in this area from most mfrs.
Beside the ISO code, the TAN (Total Acid Number) is the next most important optional test for an ATF UOA as it shows when the fluid is beginning to get acidic. The condemnation limit for TAN can be elusive because you need to know where the original fluid started. The rule of thumb I've seen is that condemnation is when the fluid rises 5 units above the starting point for ATF. If the new oil was at 1, then about 6 would be your limit but if the oil starts lower, the upper limit would be lower. Best to test a virgin sample of the oil tested and work from that.
More complicated than you figgered?
The oil is good till it's not and doesn't know the passage of time much. The OEMs don't say "lifetime" without some justification. I would set a mileage limit for tests, not time. 30K in normal service, 15K in severe. For many, that's probably too soon but you could learn something from the tests.
FWIW, my overall prescription... change the oil and filter at 5-10K, add an inline trans filter to the cooler line, use a brand name oil with the certifications for your vehicle and you will both get a long life from the trans and you'll get the most life from the oil. Go the whole nine yards if you test. Otherwise, IMO, you have wasted the money and not gained much useful info.
