Never flush an old transmission. It relies on some of the grit in the fluid to make the clutches grip. Standard pan drop and filter change is all that’s required/advised.
This video from a Toyota technician advises as such, also has plenty of other wisdom to live by if you want your car to last.
I like The Car Care Nut and enjoy his videos and his advice but I’m still sceptical about the notion that one shouldn’t flush an older auto as the floating clutch particles are providing grip that the clutches need to grab the gears (while not blocking the fine oil passages and solenoids within the box itself?).
If such was true, given all the businesses selling snake oil remedies within our parts stores, you’d expect to find a transmission fluid additive containing finely ground clutch material for added grip
I’ve seen transmission additives that claim to reduce slip. I suspect such is achieved by softening the seals which helps those one way ball values and solenoids work better and this provide the designed pressure to the clutches (none claim any form of clutch material is contained within specifically to aid clutch grip)
In my view a full flush is the way forward and if an auto then slips with the correct fluid level within, because all the clutch material has worn out and was dumped with the old fluid, the trans is already stuffed and you should be considering a rebuild
