It's useless in a manual tranny.

But if you're having difficulty with a sluggish or slow shifting A/T, particularly on cold mornings, it's likely due to varnish and gummy crap in the valve body.

It's good to change the fluid. Adding 6-10 ounces of lacquer thinner to the fluid, driving 300-600 miles, and then dumping the fluid (and with it, the lacquer thinner and dissolved varnish) can help shifting problems.

Leaving in the lacquer thinner in the tranny helps nothing because it'll soon evaporate, which means that the dissolved crap will simply reappear in the valve body.
--
Don Foster (near Cape Cod, MA)