This is from the reo oa331 engine and clutch overhaul manual:
"Using constant current control, set at 26.8 each (??? mA), voltmeter reading must be 18.4 to 19.6V with unit in water at 200f having minimum flow of 10fpm past the unit and with power on form a minimum of 6 minutes." Not easily duplicated by the DIYfer. Assuming the 26.8 is milliamps, that means 687 to 731 ohms at 200F. The best way to verify a gauge is to get a meat thermometer from your local store.
Oil pressure sender:
60lb unit
0 psi = 0.0 to 1.0 ohm
15 psi = 2.75 to 3.5 ohm
30 psi = 14.5 to 16.5 ohm
60 psi = 28.0 to 31.0 ohm
120lb unit
0 psi = 0.0 to 1.0 ohm
10 psi = 1.5 to 3.0 ohm
70 psi = 16.5 to 18.5 ohm