This question actually gets to the heart of why people in the field of fluid mechanics rely so heavily on non-dimensional numbers. It really depends on which is more important in the flow you're considering, viscosity or density (which obviously leads to the Reynold's number). read more
If it's moving relatively quickly (turbulent flow) then the density forces will dominate and oil would require less energy to pump than water. If it's moving relatively slowly (laminar flow) then viscous forces dominate (making the friction factor much higher) and water is actually easier to pump. read more