The broad distribution of NMR proton relaxation times in crude oil is a consequence not only of the overlap of a multitude of different constituents, but also of interactions of different strength with asphaltene aggregates, which was found to be influenced by the maltenes' aromaticity. The so-called dispersion, or frequency dependence, of the longitudinal relaxation time T-1(omega), and the ratio T-1/T-2 as a related parameter, becomes more pronounced for aromatic solvent molecules, in particular for fluorine-containing tracers, and is assumed to be a consequence of either or both the nuclear-electron interaction with the asphaltenes' radical component or geometric trapping of maltenes within the asphaltene aggregates in crude oil. In this work, we seek proof for this assumption by investigating the concentration dependence of relaxation properties in solutions of asphaltenes, and by comparing the relaxation properties to a number of simplified model solutions possessing either similar geometry or radical concentration. The notion of an enhanced interaction strength is further supported by dynamic nuclear polarization experiments of native crude oil doped with selected tracer molecules.