What I do every 30K is:

1. Drive X to warm tranny fluid.
2. Drain fluid from drain plug in tranny pan.
3. Refill with approx. 3 3/4 quarts fluid.
4. Drive X around to mix fluid.
5. Repeat drain and fill, replacing washer on drain plug with an engine oil pan drain plug washer.
6. Drive X around to warm fluid.
7. Check and adjust fluid level.

Some people spend about $100 for a professional flush, but IMOA it's a waste of money.
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"Half the people can be part right all of the time, and
some of the people can be alright part of the time, but
all the people can't be all right all of the time."