Although widely accepted, the phonon-mediated, BCS-like theory of superconducting A 3 C 60 fullerides (A= K, Rb, Cs) cannot reproduce correctly all their parameters, even in its strong-coupling, Migdal-Eliashberg limit. The fundamental difficulty, ascribed to intrinsically close phonon and electron energy scales (respectively at 0.2 and 0.25 eV), has been overcome by dynamical mean-field theories (DMFT), which, unlike ME, consider electron-phonon and electron-electron interactions on an equal footing. The unconventional phenomena predicted in the new framework include, among others, isotope effects on spin susceptibility, totally absent from standard theories. We have tested these predictions, finding a significant dependence on the isotopic mass in both T c and, more importantly, in the normal-state Pauli susceptibility χ P. The comparative measurement of χ P in two different K 3 C 60 samples (85% 13 C …