June 4, 2010, my partner in our 1979 PA-28-236 (Dakota) Richard Klein and I, as is common for us, left KDXR to practice some instrument approaches. I flew first and did the ILS 6 at KBDR, followed by the ILS 36 at KOXC. We pulled off the runway at OXC, and, as per our usual practice, switched pilots. We were both out on the right wing during the switch and all seemed fine. We taxied back to runway 36, planning to do some more approaches. We climbed out heading northwest. About 5 minutes into the flight, I sitting in the passenger’s seat, looked down at our relatively new JPI engine monitor and noted that the oil temperature in the right hand column was at the very top, indicating unusually high oil temperature. I told Richard who was flying under the hood and to his and my surprise, the standard oil pressure gauge read zero. We reduced power, declared an emergency and, now at about 2500 feet MSL flew back to KOXC. Luckily for us our virtually new factory reman Lycoming O-540 continued to run til we coasted to a stop and shut it off. Exiting the airplane, we noted that the entire right side of the airplane was covered with oil. Examination revealed that our newly installed oil cooler had a cracked housing that had allowed all of our oil to escape in just several minutes. No oil remained in the engine. Our engine, with only 50 hours on it needed a new crankshaft, bearings and two connecting rods. Lycoming rebuilt it for us at a cost of approximately $ 35,000. We were happy to have gotten off without a crash. The country NW of OXC is heavily forested. Had we not had the JPI installed, the first sign of our oil problem would probably have been when the engine seized. We considered ourselves lucky. The day we purchased the JPI monitor was a very good day, and it may well have saved our lives.