I actually thought it sounds like a load of old tosh as well, but I thought I would be diplomatic about the way I said it.
Fuel is not magnetic, and neither is air or water so I am not sure how it could possibly work. Bear in mind I work with magnets..
The only way it could just possibly make a extremely slight difference is if you had a superconductiong magnet in there somewhere as these are powerful enough to cause attractions in non metals and non ferrous materials. But that wouldn't work because the weigh about 3 tonnes, attract anything ferrous within about 5 meters, and require filling with a Cryogen such as liquid helium.
How a radiator additive can achieve this is beyond me. But if anyone need me to check with one of our scientists..