2003 4.7L 4WD Tundra. 165,000 miles. My ATF maintenance is as follows: Every 15,000 miles--Aisin pan drain (4 quarts & refill with Toyota T-IV or Mobil 3309). Every other pan drain--replace Magnefine in-line filter. The results have been that the fluid is a nice red color and the transmission shifts like new. This old pickup was purchased new and, at 67 years old, I do all the maintenance personally. We tow quite often, a car hauler over thousnds of miles. The pickup has a factory ATF cooler, but I really should have gotten a 3/4 ton for such usage.
These Toyota Aisin ATF's are darn good. Our 16 year old Camry V6 has 285,000 miles and with pan-drains, only, at 30,000 mile intervals with Mobil-1 ATFluid, it, too shifts like it did when I drove it off the showroom floor. Never had one bit of trouble with either transmission. Proper, vehicle recommended fluid & a reasonable maintenance schedule is the key to transmission service life.