It will only work for varnish build up .... but it is a cheap thing to try first and has helped out a bunch of cars... a couple of them mine... in dramatic fashion....
If you can suck out a couple of quarts of trans fluid from the top of the car and put this in ( less than $20 even if two cans are needed for initial application ... my 81 wagon needed that much ) it is a clean and quick attempt that does have some chance of success....