This was discussed previously. I don't know where the thread is.

Of note:

- any impurities or additions to the fuel will most likely be cleaned out by the fuel filter. Depending on the organic composition of the fuel caps, it's possible none of it will pass through the fuel injectors, either.

- any coating on the combustion chamber is bad. That's why service centers offer carbon cleaning, to remove said coating. And no "organic" coating will survive the combustion process. That's why the carbon layer forms. Impurities and unburned gas are soon turned into a layer of carbon ash or soot on the chamber walls by the heat of combustion.

- at most, I suppose, it could include octane boosters, but I've never heard of octane boosters in solid form. If it is an octane booster, you can get the same effects (or actually, a higher octane boost) by buying good gasoline.

------

Any tests on the efficiency of said fuel savers must be done by blind tests conducted over ten or so full tanks of gas. The driver of the vehicle must not know that a fuel saver is being used, or they may drive differently.

A likely testing regimen should be semi randomized, but you should do your best to ensure minimal testing condition differences. Mid-summer, heat may cause you to lose 1 km/l. Around Christmastime, you may get a boost of 1 km/l from cool temperatures and better engine operating conditions, or lose 1-2 km/l due to rain. Christmas traffic often brings fuel economy down to single digits, too.