Synchronization of pairs of serially connected Spin Torque Oscillators (STO) was studied using Landau-Lifshitz-Gilbert-Slonczewski magnetodynamic simulations. The impact of realistic process variations on STO synchronization was estimated by allowing for different in-plane anisotropy fields (H(k)) in the two STOs. We find that the synchronized state is extremely sensitive to STO process variations and can only sustain up to 4% H(k) variation. However, the addition of a small time delay dramatically improves the robustness of the synchronization and allows as much as 145% H(k) variation in the entire out-of-plane precession regime.