4l60e fluid swap
If you have been flushing or draining every 20K miles as flush is safe. If you have neglected the transmission, say 35-50k miles between fluid changes, then if you do a flush and the converter is full of sludge, the tranny will start slipping after some time.
I'm not trying to scare you, I know from first hand experience. My prior car (84 TA) I did not maintain the tranny very often. This car that I owned for 6 years had a flush with no problem what so ever. I also added a B&M trans cooler for safety and keeping the fluid safer, for a $100 it is worth it. Most dealers would rather flush because of cost and it is easier, saves on dropping the pan and prevents pan leaks.
Hope this helps and this is my opinion from experience.
If you change the fluid with any regularity... and you should if you are driving it hard at all... then changing the fluid every 15,000 - 20,000 miles would be just fine. Be sure to change the filter regardless of which service you perform.
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