Drop the pan. That usually requires that you lift the engine off the motor mounts a few inches. However, low oil pressure usually doesn't mean a bad oil pump. Typically, low oil pressure means that the engine main and rod bearings are worn. Engine rod and main bearings do not ever touch the crankshaft; they ride on a film of pressurized oil. When the bearings wear, the oil pump is no longer able to generate enough oil volume to keep the pressure up. One way to determine if it's oil pump or bearings, change to a heavier grade oil (higher viscosity number) and see if the oil pressure stays up after the engine warms up.