These questions are answered in the 9-2320-280-10 Operator's Manual. Going by the limits, the engine oil pressure should be "more than 6PSI" once warmed up at idle and "more than 15 PSI" when under load, but it shouldn't really exceed 50 PSI. I feel as if every auction HMMWV idles at 25-30 PSI.
"Oil pressure at idle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 psi (69 kPa)
Normal operating oil pressure . . . . . . . . . . . . . . . . . 40-50 psi (276-345 kPa)"
Operating temperature should be above 190F but below 230F. Don't trust your gauges for these values until you manually check it against an external gauge.