It is theoretically possible to lose prime if the lines are off long enough. The early engines with the distributor driven oil pumps can have this happen. The newer style front covers used with the serpentine belts where the oil pump is crank driven do not have this problem. I have had a couple of engines lose prime during various repairs such as timing chain replacement...even oil changes if the oil is allowed to drain long enough with the filter off, too. To reprime, I remove the pressure relief valve and fill the oil pump with prelube...as much as you can cram in there ...and fill the filter with oil, too.