An oil pipeline was constructed to transport an oil with SG = 0.8 for a distance of 10 miles. The country was hilly, so the line made many ups and downs. These may be considered equivalent to 10 rises of 200 ft, followed by descents of 200 ft. When the pipe was completed it was tested by pumping water through it. The water flowed satisfactorily with an inlet pressure of 150 psi. Then the oil was slowly fed into the pipe. As the oil flowed, the pressure at the inlet end began to rise, and the flow rate began to fall. Finally, the flow stopped all together, while the pressure at the inlet side remained at 150 psi. Explain what caused this.