1998 Nissan Pathfinder hard to start

Engine Performance problem
1998 Nissan Pathfinder 6 cyl Two Wheel Drive Automatic

Hi my sons pathfinder has starting probs, runs great when started. He uses starting fluid to start. Problem started when he was 2 qts low oil filled everything ok. One week later prob back changed oil pressure sender no help, changed oil and filter and sender again no help. Could it be fuel pressure?How is fuel pressure tested?