Engine Restorer test
 

So, I would be reticent to use this stuff, having used stop leak in the past and been disappointed. However, this "Project Farm" report on a longterm test is pretty convincing that this stuff could restore some useful power in a genuinely ailing engine. Pretty great channel, too.

Engine Restorer one year later:
https://m.youtube.com/watch?v=VXIu3oo8z4c

restorer
 

Is it something added to the crankcase ? My 1958 Morris Minor 1000 used more oil than fuel, and really ran on STP, which increased oil viscosity so much, that the oil probably struggled to make it to the oil rings in the first place!:p

Added to the engine oil and seemed to seal the rings a bit better. In his test, compression improved after a few hours and then more over the course of a year.