It is shown that the magnitude of the streaming potential (V-s) can be significantly enhanced from similar to 0.02 V to as much as similar to 1.6 V, in electrokinetic flows through microchannels. This was done through flows on liquid-filled surfaces, where the grooves were filled with oils of viscosity in the range 30-3000 mPa.s. The presence of immiscible oils and the improved slip are both factors that could significantly increase the V-s. The analytical relationship between streaming potential and filled liquid viscosity was derived and verified through corresponding experimental results. The work yields novel insights into complex electrolyte flows and indicates avenues for more efficient energy harvesting.