This is an old writeup I did after flushing an auto in an '86 Nissan V6.....
I did several flushes over say 10K miles using regular ATF and each time checked the fluid coming out in a clear jar...I could see floating "gum" particles. After about the 3rd flush I put the synth in along with the Lubegard additive...which is what finally helped a sticking valve in the trans.
You can find the Lubegard add at NAPA...but you need to use the exact type called for for your car.
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My experience (which is limited) is that when you change or flush an auto, you remove the buildup of clutch grit and metal particles (which are wearing your trans out), but you also put in new ATF which starts to pull the varnish and gum out off of the interior surfaces, valves, etc...and with the grit gone, you can have more slipping.
Best bet is to totally flush the trans thru the hoses at the rad...this way you can remove almost all the ATF from it and replace it with new...instead of about 1/2 of it thru the pan.
You would probably want to start with regular ATF and do a full flush 2-3 times lasting about 1000 miles of drivng each....in order to clean out most old sludge, grit, and varnish......and then put in Mobil 1 synth ATF.
Mobil 1 synth usually helps a trans to have crisp shifts, if it's not loaded down with crud.
The vacuum modulator valve also controls shifting in a major way and my limited experience says it would be best to use a used OEM vac modulator rather than a new aftermarket one. New OEM is even better.
How to do it: Check that the trans is at a FULL level first.
If you've removed or drained the pan, you've probably removed about 1/2 the ATF already & cleaned the pan & replaced the screen or filter.
With the front of the car on ramps....find the hose at the radiator that comes from the trans (one that flows to the radiator), put the radiator end in a bucket...& with a hot trans...run the car at IDLE in PARK till the flow almost stops...then shut it off.
Add the same amount as you took out. Do this until you see clean new ATF coming out. If you have a clear container you can compare old & new ATF.
Put the hoses back on securely and check your ATF level. Drive for 1K miles.
* Need to be very careful when removing the old hose from the radiator...or you might end up replacing the rad. You also might want to replace the old hoses if they are hardened...but use ONLY the right hose type though. With new clamps.
Later, if you do the flush a couple of times without removing the pan...you will need to exchange all of it.
If the pan gasket did not leak, and it comes off with the gasket intact...just bolt it back on to specs...do not overtighten the bolts.
This flushing method is easier on the trans than the commercial methods, since it uses the regular working of the trans to do it. Keep things very clean...no lint from a rag even.
I also use Lubegard trans additive which can help with sticking valves and governors that synth ATF might not help...you need to buy the right kind for a Honda though.
Redirecting...
Also sell a flushing additive....
I do a total flush about every 15K miles with Mobil synth ATF after cleaning the pan. And add more Lubegard.
Another method is to drain the trans pan at each motor oil change and refill, replacing about 1/2 of it. This would be done after the flushing process was done.
Some trans have bands/clutches that can be adjusted, but if it works reasonably OK...I'd say, just flush it and don't mess.
* no guarantees....this process may or may not work as expected....all transmissions are different....so are the people working on them....