I think the old fluid in a high mileage tranny is a domestic car thing, our Aisin transmissions are a different kind of monster. I have 170k on mine and it was flush and filled with Diamond sp-4, my other one has 130k and had probably never been changed until i owned it and i did the same flush and fill with the mitsu trans fluid. Both are solid and have no problems, it won't hurt to have clean fluid. Aisin transmissions like clean fluid it keeps the hydraulic valves clean. LT learned this in his 89 recently, his filter clogged and his tranmission would boil over after driving it a few miles. He put in a new filter and fluid and it was good to go.