Disclaimer; works for me, proceed at your own risk.
DADS50, I've read of the technique you mentioned. The better system is exactly what SimS mentioned. I usually disconnect the transmission fluid outlet line (either at radiator or transmission, whichever is most accessible) slip on a section of clear plastic hose (from Lowes) so I can see what's going on while the hose dumps in a 5 gallon bucket. Then crank the vehicle, let it run at idle until the fluid flow starts to slow. Kill the vehicle, pull the pan, clean the pan and magnet and change the filter. Then add the factory specified full capacity of transmission fluid. Crank and watch the clear hose and you'll see the last of the dark dirty fluid change over to clean new fluid. Shut the engine off and reconnect the transmission fluid line. Check fluid and top off. Then drive till warm and recheck per the manufactures recommendation.
The above has worked for me on many vehicles, including trucks, cars, imports and domestic. I try to do at 50-75k intervals.
added in edit; If I acquire a vehicle that has a lot of miles on the transmission I always add Lubegard after a fluid change. Lubegard Red for GM vehicles, others I go with Lubegard's recommendation. Reason; with high mileage vehicles still running the original fluid, the near microscopic ground up transmission band and clutch wear particles with their added coefficient of friction are often all that's keeping the transmission working. New clean fluid can cause worn bands and clutches to slip, transmission shifts can go from OK to terrible or not at all.
A quality additive with proper friction modifiers can help and sometimes eliminate a shifting problem. In my experience at least. Of course it's not a new rebuild in a can but it can help a weak tranny run for a few 10s of thousand miles.
Last edited by moparguy; Sun Mar 15 2020 07:50 PM.