I have an early 2000s Subaru with about 130k miles on it. I started encountering a rough idle in the spring of 2018. After trying a few usual suspects to no avail (cleaning the throttle body, changing the fuel filter), I shrugged and poured a can of Seafoam into the tank before filling it up. Result 1: the rough idle disappeared (though this could have been a delayed effect of the other treatments finally kicking in). I was more impressed, though, by Result 2: my rate of gas consumption dropped 2.5 MPG, enough that I was using over a gallon less per week for the same commute. The can paid for itself in two weeks. I'm almost inspired enough to try the other advertised applications (putting it in the crankcase, or in the oil before a change).