Only works on the really worn out stuff...like 200 000 miles. I've tried it and I was impressed how it restored the 2.8 V6 I had in my Bronco-II (375 000kms). Until 2 weeks later it seemed to fizzle out and the blow-by returned to the way it was. Another can freshened her right back up though, it gets expensive quick obviously...I think I prefer Rislone which is like STP to help cure excess blow-by. Probably a good trick if you wanted to sell a car. I later tried it on a good engine and couldn't tell the difference. When I later rebuilt the engine there was a silver liquid in the bottom of the pan that looked a lot like mercury but wasn't, the stuff out of the can is blue so I don't know what the heck the silver stuff was. Didn't seem to hurt the engine any?
Basically if you need this stuff you need a rebuild, save your money.
“She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.”
— Han Solo