Only one coating that does works for me. I had after 115k miles a yowling 5th gear in MT and also such diff. Both are fine afterwards and since ~62k miles. To the present.
The important trick is:
You should put it in OLD oil. If a one put it in 5k miles old diff oil, that dont works so fast (or, rarely sufficient good) as if you put it in ~37k miles old oil.
Similar with engine oil. Works much better in 7000 miles old oil than in oil with 3000 miles.
Put it in warm/hot oil, drive ensuing ~1/2h the car. Afterwards, drive as always. The process take ~1200 miles.
For the "why don't they do it from the factory" buffoons:
Coating with silica_any stuff works only during functioning and is a longer process. Shall GM recruit 1000 trainees which drive 8h every day colon with 1000 brand new cars around the factory?
Therefore similar but acceptable solution for manufacturers is named: plasma coating. This works instantly in few seconds, but is very expensive. Advantages: you can coating metals with near anything you can vaporize (from solid directly to gas/plasma). Also with titanium & co.
Thats a no-fake few scientific researchs in the web about coating metals with silica_any, with pressure + friction + heat. Go visit Aunt Google to be littlebit smart.