My understanding is that changing old fluid with new fluid with dislodge and plug up the transmission. That was my mistake with this one. I got the car off a friend and got it running. In an attempt to service the transmission I did a flush with about 230,000 miles on it. The old fluid was old and brown. Before the flush the transmission was running great and really smooth. Then it started to slip after flushing it. I believe another person I spoke to also received the same advice from a friend who worked on cars. Once the transmission has been allowed to be driven without proper service its best to leave the old fluid. My other experience with a 90 245 I put back on the road a few years ago and my brother drove 100,000 in 2 years was its best to just drain the pan every 3-6 months and refill rather than a flush all at once. That transmission was horrible when I got the car. But with some additive and draining the pan and actually allowing the thing to bleed through a leaky tailshaft seal for a while it has survived to 330,000.

My belief at the moment with the autos is that if you swapped out the valve body with a toyota that already has a metal check ball and you slowly replace the fluid over a couple years then the AW70 (the weaker trans of the two 70/71) will last beyond what the FAQ section states they last to about 250,000.

The key is to not allow the transmission to start slipping or otherwise the clutches will go bad. If you can maintain the valve body then the clutches would last maybe 350,000 to 400,000.