Yes, I saw the Fitch Fuel Catalyst on TV ... probably a show on the Speed Channel.
Looks like pure BS to me. I don't understand how the fuel can be changed (or the catalyst would be used up in a tankful or two).
My guess is that all it does is filter the gas ... which is what your fuel filter is supposed to be doing.
Fuels that start in a gaseous form
should burn more completely as they are pre-atomized. The problem comes with the size of the fuel tank/cell and the (lack of) ease in handling the fuel as it often has to be under significant pressure.
Yes, most cleaner-type products that advertise the ability to improve fuel economy are simply restoring some ability that has been lost due to fouling of the injectors, valves or other parts of the fuel system.
Very few claim enhancements over a clean engine ... but there are a few. Some of these are Marvel Mystery Oil (MMO), Lucas UCL, Schaeffer Soy Neutra 131C and a few others. These claim to increase the
lubricity of the fuel which would reduce the parasitic drag of all engine parts that come into contact with fuel ... namely the piston rings.
I have used these and for a month-long run, found that MMO may have worked to increase mileage as much as 5% over 3-4 tankfuls. Have any old 2-cycle oil laying around? Try adding that to your fuel ... so that you end up with a 200-250:1 mix ratio.
Whenever I try these adds, I always give my engine a break after a few tankfuls in case and of this stuff is building up where it doesn't belong. For a quick cleanu-up, I use either Chevron Techron or Red Line Oil SI-1 in modest doses. No need to add more that 2 bottles of these powerful cleaners to your tank per 15,000 miles.
Something else to consider is simply switching to a thinner oil in the motor and trans to reduce hydrodynamic drag. Try a 5 or 0W-20 in the motor and Pennzoil Syncromesh in the manual transmission. Just be aware that these MAY accelerate wear ... though I have not seen proof of this, in cast just the opposite: UOAs (Used Oil Analysis) shows that 0W-20 protects very well ... even in motors calling for 5 and 10W-30.