We report an analysis of data from Nb/AlOx/Nb tunnel junctions. We show that both conductance and subgap leakage follow simple scaling laws as a function of oxidation conditions. The leakage effects are discussed in terms of a simple physical model of tunnel barrier inhomogeneity that is also applicable to directly deposited barriers on NbN. We demonstrate that an improvement in barrier properties can be achieved by the use of a buffer layer overlying the barrier, and that this improvement is consistent with a model based on resputtering effects.