I'm a little reluctant to reply, but I will anyway. Friction modifier is something that is blended into normal transmission fluid. Some types of trans fluids have higher levels in it than others. An example is Chrysler ATF+3 and ATF+4 has a higher level of friction modifier than Dexron III/Mercon. It is part of the additive package. Just like in engine oil, when the additives get depleted the fluid no longer does it's job as well as it can.
When the torque converter shudder problem was first emerging with the Ford transmissions (Crown Vic, Grand Marquis, Town Car, Thunderbird, Cougar) about 1994 I had a Ford hot line engineer tell me that they had been recommending adding friction modifier to cars with that problem and that many cars were fixed by it but that they had been told to stop making the recommendation. He said that if I were to do so, use one 4 oz bottle of Ford Friction Modifier per car with a pan drop fluid change and not to show the friction modifier on the warranty claim. I did so, and it worked. On several cars. They did not return. It seemed to be a permanent repair.
Shortly thereafter the official fix depended on the miles on the car and the severity of the shudder, but with most cars getting a new torque converter. Eventually the official cure became to flush the trans with a fluid exchange machine and that was all that was needed. The TSB published about it said that the cause was "worn out fluid". My take on that is that the friction modifiers in the fluid were depleted.
I will absolutely agree though that if you are not a lube chemist you will likely throw off the additive balance by such experimentation. To me, the preferred answer is to change the fluid. A 100% fluid flush or exchange.
On the topic of brake fluid added to transmission fluid. This is something that has been around for decades. Brake fluid might stop a transmission leak. It tends to swell the seals. I would again prefer to try fresh fluid first and then if the leak persist and you don't want to do a tear down and really want to try some snake oil, buy a snake oil that is specifically intended as a transmission sealer. Sometimes it works, sometimes it doesn't.
