Rheology experiments where oscillatory shear with high frequencies or large amplitudes is applied often results in the appearance of shear thickening or shear thinning phenomena. The dependence of the viscosity as a function of shear rate generates higher harmonics in the shear response. Polyisobutylene allowed experimentally the spectral analysis up to the 21st harmonic contribution. This rich information permits detailed characterization of the material in the non-linear regime.