This is going to sound more churlish than I intend, but...
I wonder what the true percentages of success are for the various stop leaks? Whether they work or not (and in my experience they don't), everybody says, "Oh, might as well try the cheap route first." That "forlorn hope" might be all that's keeping some of the "magic elixir" companies alive... all those desperate people. No doubt some will chime in with some story or other, but my guess is that the vast majority of product is a waste of money, either from it being a product that doesn't work or the problem being beyond what any product can "cure."
In 20 years as a tech and another 20 as a "retired" tech that's still elbow deep into things automotive, I can count the times I've see stop leaks work on the thumbs of one hand. The only time I saw a "magical" result was way back, when the trick was to pour a quart of brake fluid into the engine just before an oil change, run it a while and then dump the oil. Worked on a VW air cooled whose rear seal had gotten rock-hard from heat (a common problem back then, mostly due to the materials). Ended up having to replace it anyway later, though. Heck, as easy as a Beetle engine was to pull, it was the height of laziness to not to do the repair (I think my record was around 5 minutes to pull a type 1 engine... so call the dog in and see if he's wet).
Before you think I'm riding the high horse, I'm just as guilty of this "hope" stuff as anyone. I recently tried to solve some trans leaks on my Farmall tractor with stop leak... the real deal... an ester-based seal swell/conditioner compound.Ran it six months. It might have slowed a little but I had to still go in and replace the seals. Maybe they were softer but there is little effective difference between a hard worn-out seal and a soft worn-out seal. They both leak.
