An attempt to understand the microscopic origin of the high viscosity of Brazilian heavy crude oils was made combining macroscopic (theological measurements) and microscopic [small-angle X-ray scattering (SAXS) measurements] techniques. A clear relationship between the asphaltene content and viscosity was found, while the removal of asphaltene via flocculation led to a large viscosity drop, confirming them as the origin of high viscosity. The SAXS analyses of crude oils confirmed the presence of asphaltene aggregates as fractal-like particles of colloidal dimensions. Afterward, a systematic investigation was performed on the effects of a series of additives and physical treatments on the crude oil viscosity. Physical methods did not cause any significant viscosity drop as well as more than 80 additives tested. SAXS measurements on oil samples containing toluene and heptane indicated little effect on the asphaltene nanoaggregates within the dimensions probed by SAXS, confirming a general mode of action based on aggregate dilution instead of disruption.