The spreading of low molecular weight silicon oils (PDMS) on heterogeneous substrates has been studied by spectroscopic ellipsometry. Using different grafting times, we produced partially OTS-grafted substrates whose surface energy ranged from that of the bare silica surface (about 28 mN/m) to that of the completely OTS-grafted substrate (about 21 mN/m). Conversely to what has been observed in the case of homogeneous substrate, the effective diffusion coefficient of the precursor film decreases monotonically as the grafting becomes more and more effective. Results of Molecular Dynamics simulations of a polymer-like liquid spreading on a heterogeneous substrate are presented and support the experimental data in the case of regular patches of heterogeneities.