Did a Transmission oil and filter change today. OEM filter and gasket with Amsoil ATF. I flushed a little over 12 qts out of there and added 12 qts back in. I 1st disconnected the cooler return line and ran the truck to pump out the old fluid and drain the pan. I got about 5.5 qts that way. I dropped the pan and had about another 1/2 qt or so in the pan. It was actually cleaner than I was expecting. I cleaned out the pan and magnet and the gasket surfaces, installed the new filter, and put the pan back on and torqued it to 80in lbs.
I refilled about 5.5qts and continued to flush out the old fluid. I took about 4 "purges of adding and flushing until I had 3 gallons of mostly old fluid and it was coming out bright red in the poly line. Shifted through the gear while idling and drove it around town for about 20min. seems to shift great, very smooth. I am just above 48K miles in the truck. Its not too warm in Indiana, had a hard time getting the truck above 120 degrees. I guess the Amsoil does its job well! I just made sure the ATF was at the top of the cold mark when running. I'll continue to check level and add if needed later.