There is a good video from ZF technicians about this topic. ZF is a German manufacturer of automatic transmissions that builds automatic transmissions for BMW and Mercedes, for example.
They strongly advise against using additives when flushing automatic transmissions.
Firstly, only the abrasion of the friction disks accumulates in an automatic transmission.
This will be flushed out by the flush anyway, because it is floating in the oil, and if residue remains somewhere in the gearbox, that's just the way it is.
Additives, on the other hand, that have a cleaning effect can no longer be removed from the transmission without leaving any residue, because there are simply too many valves, oil channels, pistons and so on.
From my point of view, a transmission flush is actually just “a transmission oil change” anyway.
Because this is not a flush that is supposed to remove any dirt from the transmission.
The problem with automatic transmissions is that you can't get the old oil out of the transmission without flushing.
That's why you usually connect a machine in a workshop that sucks out the oil on one side and fills in fresh oil on the other side until the machine notices that clean oil is now being sucked out.
The oil is now replaced.
These flush units make sense because they are a reliable process for the workshop and don't involve any mess with gear oil.
Additives are not necessary.
You can flush your transmission in the same way if you disconnect your hose from the top of the transmission oil cooler, attach a hose there, lead it into an oil pan and let your engine run briefly.
Then you stop, fill up the amount of oil that came out from the unit, again and repeat this procedure three to four times until the oil is changed.
The quality of the oil change is on par with a professional flush.
