For the oil pump to suck the pan dry at highway speeds, he would have to be turning in excess of 5,000 rpm at "highway speeds". This would mean gears of at least 4.88:1 or lower, and highway speeds in excess of 70 mph. The relief valve sounds like a more likely candidate for a mystery oil pressure loss, as it happened hot, and did not go away immediately upon bringing engine speed down. I would disassemble the valve, check for nicks and burrs, and clean it out. In the last few years, assembly quality has gone down a little, and some pumps get assembled with shavings in the relief valve passage- this can cause a sporadic loss of oil pressure, especially after a spike of high pressure (which would move the valve).
