The second website you posted
http://www.fuelsaving.info/ffi.htm
Made for pre-1973 cars initially to run on unleaded fuel.
Supposedly coats the cylinder walls with something to promote full combustion. What can withstand the immediate temperatures of the combustion to do this. Its not metal that would risk ring or wall wear. Teflon maybe, I think teflon is good to about 500 degrees F so it could work. I don't see this making enough of a difference to affect anything though.
It also increases the octane slightly like I guessed in the last post. That is probably the only increase in mileage people are noticing. Either way without more information I am pretty convinced against this product. By more information I mean lab tests by a third party.
Worried about gas mileage? Do your oil change regularly, tune up your car, make sure the sensors are within ideal ranges with a computer diagnostic, change your fuel filter, make sure your tires are not underinflated, don't carry more in your car then you have to, don't stomp the gas pedal down, get a high flow air filter (K&N are good ones for example.)