We study the asphaltene microstructure in a heavy crude oil in response to the addition of solvents of different qualities by measuring the intrinsic viscosity and the Huggins coefficient. Dilution with good solvents (xylene and cyclohexanone) causes the native asphaltene clusters swelling, while that with poor solvents (a diesel oil and a C11-C14 alkanes mixture) may induce different microstructural rearrangements. The mixture of C11-C14 alkanes induces the aggregation of clusters, while the diesel oil their shrinkage. Moreover, if the diesel is added in small quantities instead of cluster shrinkage it may induce cluster aggregation thus forming larger "macro clusters". These structures survive after further dilution in both poor and good solvents and their formation is always accompanied by a reduction of the Huggins coefficient that implies the better solubility of these self aggregated structures in the used solvent. Finally, an unusual negative Huggins coefficient is measured, when the macro-clusters are formed and the solution is diluted in a good solvent. This might be due to a prevalence of the infra-macro-cluster interactions on the inter-macro-cluster ones.