Let's be honest here; if your car is running like crap nothing added in the fuel will fix it. Think of these products as preventative maintenance. There are several products on the market that advertise that you should just add it to your tank to magically clean the fuel injectors. Other products say to add it directly to the crankcase to get the gun out of the oil system. In my experience, most of these products are an expensive sham. You'll be surprised which one does the job!
I have personally used many chemicals in my engines with several varying results mostly erring on the side of ineffective. One such product actually seemed to make a difference. That's right, in a parts store with an entire isle dedicated to engine additives, only one product sitting on the shelves is worth the cost. The only one that seemed to actually do what it says it does; Sea Foam Motor Treatment.
SeaFoam is an awesome product that definitely works. I have personally seen several great results from the usage of this additive. Putting it in the oil helps clean out that pesky oil sludge that builds up inside of an engine. The proof is pretty evident once you drain the oil out. Putting in the gas tank helps clean out the stuff that gets stuck inside of your fuel injectors, and helps clean out the carbon buildup on the back of the valves on port fuel injection. Those with stubborn lawn equipment can use it to clean out fuel deposits in carburetors from ethanol based fuels. Injecting it through a vacuum port in a controlled manner essentially allows it to clean the intake manifold, the tips of the injectors, and the back of the valves.
If done correctly, it is basically a fuel service in a bottle. I kind of shy away from using it in my oil, as freeing up too much sludge caked on the side of the engine can cause oiling issues. Also, if used in the oil, put it in and drive around for about an hour then change the oil. I really don't recommend leaving it in the oil or read my other article about Professional Fuel Injection Cleaning.