Marvel Mystery Oil
I've been using Marvel Mystery Oil as a gas additive for the last month or so - I'm really liking the results.
For those who aren't familiar with it, MMO's kind of an old timer's product. You can use it as either an oil additive, or a fuel additive. As a fuel additive, it's an upper cylinder lube, fuel pump lube, and has some injector cleaning power. It used to come in a little black/red can, though it's in a plastic bottle now. About $5 for a pint at Autozone.
While I haven't used it as an oil additive, and probably won't, the fuel additive aspect is what I can personally speak to and recommend. The instructions suggest 4 ounces/10 gallons - based on my experience, and research at BITOG, I've found that 4 ounces/tank is best for my car.
I've noticed that the car is significantly smoother, RPMs seem to build faster, and I've picked up between 1 and 2 mpg. I'm distrustful of my butt dyno, so I leave the power perception as "subjective", but the MPG pickup is sheer quantitative.
Also, it's often easy to get caught up in "smoother" discussions, but it's really very apparent in my vehicle. At idle, it's sometimes difficult to determine that the car is turned on. Also, I have a slight rattle in my dash that I call my "smooth-o-meter" , which is gone now. (incidentally, that same rattle indicates to me that pennzoil platinum is smoother than mobil1)
Some people on BITOG were concerned (on principle) that it could foul exhaust components, but there's not really any empirical evidence for that, and the Marvel company says they use very low ash components. I also have my own acid test, in using MMO as a weapons lube in my AR15 rifle - if I lubricate my bolt with it, it burns cleanly off, with no carbon deposits. Only 2-cycle oil burns as cleanly.
Anyway, it's $5 for a bottle, there's no harm, and I find noticeable improvements in my car. Give it a shot.