You do have to be careful, i have heard horror stories about older vehicles using it, and had some personal experience. When they have been around the block a while they have lots of crap in the engine, and over time that crap becomes an integral part in holding things together and sealing other things that have since worn out their normal usefulness. i ran it through my Equinox about 3 months after we got it and it ran better than when we got it, so i decided to try it on my 97 cavalier that has 200k miles, now i have an oil leak somewhere. Now i don't know if the two are connected, but it is awful fishy timing for the leak.
The really cool part about it is that you get to create a huge cloud of smoke from your tailpipe. Whether it is good for the environment or humans for that matter, well i don't know that, but it is neat. And if you have a downward facing tailpipe, you can see the results of your work as a nice black soot stain on the driveway
