My '85 F250 4 x 4 C6 has 270,000 miles on it and is fine. I have used Red Line D4 full synthetic ATF since 1998 when I bought the truck at 140,000 miles. Not long after I added an external transmission fluid filter in the hot fluid line from the transmission to the radiator cooler AND a big auxiliary transmission cooler in front of the AC condenser. I change fluid and external filter at no more than 30,000 mile intervals. The external transmission filter adaptor uses a WIX 51515 XP or equivalent. I have never drained the torque converter but early on did several fluid changes at short intervals.