Hello Carol,
I have found the information provided on the Car Care Nut YouTube channel to be excellent. If you haven't seen it, Amhed is a Toyota Master Diagnostic Technician with many years of experience in a dealership, and now as an independent. I am aware of several videos where he talks about changing the transmission fluid. Basically, if it has under 100K you can do a drain and fill (not a flush). You are right about the "sludge"; if you haven't had a transmission service before 100K then the transmission may start slipping if you flush it and get rid of the sludge. I love his series on maintenance.
Cindy
