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I'm not advocating them in oil, but have used them with known dying automatic transmissions and bought some time that way. The transmissions shifted quicker and performed dramatically better. There was no "placebo effect". In all cases I assumed dying pump and/or weak internal seals.
I haven't tried brake fluid to swell seals, since I don't have a junker handy at the moment, but have heard of that as a temp remedy for dying trannys too. Like running STP or gear oil in a dying engine, this isn't presumed to "fix" the problem for good.
I'm gonna see about getting a hold of a shot trans or one someone needs rebuilt but cant spend the money and teaching myself to rebuild one and learning more about them.
Depends what you want, you could probably get out with a cost as little as $1000-1200 for a cheap stock rebuild at a local shop. If you can drop the tranny yourself, you could save an additional several hundred.