Volvo s80 transmission 4t65ev doesn’t have an oil drain plug. Every automatic transmission has oil filter and on most of them it has to be replaced at some point of time.
I did the tranny flushing according VADIS instruction with disconnecting oil return line and attaching transparent plastic tube on it and so on. I used even more then 12qrt oil.
As far as I know, the Volvo’s Transmission Control Module counts the hours the tranny was working while the oil temp is above the normal, trying to predict when the oil’s going to go bad and sending a message “trans oil change” before that happened.
When oil and/or tranny oil filter is changed, TCM has to be reset in order get the oil timer zeroed and to start new gear shifting adaptation cycle.