I have a Ford Winstar (1999, 55 K miles)
I do not drive it very much: 2 to 3000 mile a year How often would you change the automatic transmission fluid assuming very mild driving conditions, but in Texas ?
I would change it according to what is recommended in the Owner’s Manual/Manufacturer’s Maintenance Schedule.
For instance, if the manual states a transmission fluid change interval of something like…“Every 3 years or 36,000 miles, whichever comes first”…I would recommend that you change the fluid every 3 years, as a result of your low mileage per year. If it recommends “Every 4 years of 50,000 miles”, then have the fluid changed every 4 years–and so on.
What does your Owner’s Manual/Manufacturer’s Maintenance Schedule state, and why do you not want to follow the advice of the people who designed and built your vehicle? Those who ignore the maintenance schedule invariably spend more in the long run on unscheduled repairs, as compared to simply maintaining the vehicle according to specifications.
Agree with VDC, unless you are towing a trailer or carrying extremely heavy loads. In that case, your manual will have something like 12,000 miles or so.
“why do you not want to follow the advice of the people who designed and built your vehicle?”
… because I am used to thinking (critically) for myself 
So the idea that came to mind was: since in my case the fluid change was likely not mileage dependent, what are the factors that affect the stability of the auto-transmission fluid? (this is actually my first automatic vehicle)
So, I read this “opinion”:
in http://autos.yahoo.com/maintain/repairqa/transmission/ques123_0.html
"For optimum protection, change the fluid and filter every 30,000 miles (unless you have a new vehicle that is filled with Dexron III ATF which is supposed to be good for 100,000 miles).
Why Atf Wears Out
An automatic transmission creates a lot of internal heat through friction: the friction of the fluid churning inside the torque converter, friction created when the clutch plates engage, and the normal friction created by gears and bearings carrying their loads.
It doesn?t take long for the automatic transmission fluid (ATF) to heat up once the vehicle is in motion. Normal driving will raise fluid temperatures to 175 degrees F., which is the usual temperature range at which most fluids are designed to operate. If fluid temperatures can be held to 175 degrees F., ATF will last almost indefinitely – say up to 100,000 miles. But if the fluid temperature goes much higher, the life of the fluid begins to plummet. The problem is even normal driving can push fluid temperatures well beyond safe limits. And once that happens, the trouble begins.
At elevated operating temperatures, ATF oxidizes, turns brown and takes on a smell like burnt toast. As heat destroys the fluid?s lubricating qualities and friction characteristics, varnish begins to form on internal parts (such as the valve body) which interferes with the operation of the transmission. If the temperature gets above 250 degrees F., rubber seals begin to harden, which leads to leaks and pressure losses. At higher temperatures the transmission begins to slip, which only aggravates overheating even more. Eventually the clutches burn out and the transmission calls it quits. The only way to repair the damage now is with an overhaul – a job which can easily run upwards of $1500 on a late model front-wheel drive car or minivan.
As a rule of thumb, every 20 degree increase in operating temperature above 175 degrees F. cuts the life of the fluid in half!
At 195 degrees F., for instance, fluid life is reduced to 50,000 miles. At 220 degrees, which is commonly encountered in many transmissions, the fluid is only good for about 25,000 miles. At 240 degrees F., the fluid won?t go much over 10,000 miles. Add another 20 degrees, and life expectancy drops to 5,000 miles. Go to 295 or 300 degrees F., and 1,000 to 1,500 miles is about all you?ll get before the transmission burns up.
If you think this is propaganda put forth by the suppliers of ATF to sell more fluid, think again. According to the Automatic Transmission Rebuilders Association, 90% of ALL transmission failures are caused by overheating. And most of these can be blamed on worn out fluid that should have been replaced.
On most vehicles, the automatic transmission fluid is cooled by a small heat exchanger inside the bottom or end tank of the radiator. Hot ATF from the transmission circulates through a short loop of pipe and is thus “cooled.” Cooling is a relative term here, however, because the radiator itself may be running at anywhere from 180 to 220 degrees F.!
Tests have shown that the typical original equipment oil cooler is marginal at best. ATF that enters the radiator cooler at 300 degrees F. leaves at 240 to 270 degrees F., which is only a 10 to 20% drop in temperature, and is nowhere good enough for extended fluid life.
Any number of things can push ATF temperatures beyond the system?s ability to maintain safe limits: towing a trailer, mountain driving, driving at sustained high speeds during hot weather, stop-and-go driving in city traffic, “rocking” an automatic transmission from drive to reverse to free a tire from mud or snow, etc. Problems in the cooling system itself such as a low coolant level, a defective cooling fan, fan clutch, thermostat or water pump, an obstructed radiator, etc., will also diminish ATF cooling efficiency. In some cases, transmission overheating can even lead to engine coolant overheating! That?s why there?s a good demand for auxiliary add-on transmission coolers. "
So, the take home message here for me was : "As a rule of thumb, every 20 degree increase in operating temperature above 175 degrees F. cuts the life of the fluid in half! " and since it is hard to tell what temperature exactly the fluid reaches, it seems to me it is crucial to keep an eye on the physical condition of the fluid itself, and of course keep in mind the kind of driving one does.
In my case the car has been driven in the mildest conditions imaginable for about 5000 miles, I even avoid traffic. It has been parked in the shade 99% of the time and Texas temperature does not exceed 100 degree F even in summer here. The cooling system is up to snuff.
The Hayes indicates 24 months - 30K miles. My on board manual indicates nothing.
I bought the car 4 years ago (used), and immediately changed - flushed the transmission fluid (whether it needed it or not). I have checked the appearance of the fluid every 6 months or so (using a small transparent test tube) and a “blotting” a drop on a paper towel : color, smell, viscosity are the same as original fluid. Shifts very smoothly (obviously).
However I will replace the fluid this year, after 4 years. When I move back to Washington State and do a lot of mountain driving I will likely accelerate the maintenance over the “recommended” schedule.
We have had a lot of discussion on trans fluids on the Sienna Club. It seems there are as many ideas on what to do as which woman to marry, heh, heh.
A few years ago, a guy had a new luxury car, and said he was going to pull the plug and drop three quarts (? not sure ?) instead of flushing etc. every time he changed the motor oil. The cartalk brothers at the time said, yes, that would be adequate.
Some flush. Some say flushing stirs up yukky stuff. Some drop pan and clean. Toyota dealers suck it out the dipstick tube. I like to drop plug and replace the three quarts. I have towing package, and color never changes.
I do worry about that O-ring problem which allegedly causes premature failures in the Siennas, especially usually being in rural Mexico. Guess I have to take my chances unless I get better information.