A fuel additive is formed from dispersions including an organic phase, at least one amphiphlic agent and solid objects based on particles of an iron compound in the crystallized form with a small size, as a fuel additive. The particles have an average size DDRX of less than or equal to 7 nm measured by X-ray diffraction, and at least 80% by number of the particles have a size DMET of less than or equal to 7 nm measured by transmission electron microscopy.