1979 Dodge Truck oil pressure

Don't let the mileage scare you. Truck is on it's third engine. Currently use Castrol 20W-50 and change oil and filter regularly. Oil pressure gauge reads 50+ when 1st crank. Drive to work on highway and gauge reads 75. Longer I drive the lower the pressure gets, however it never drops below 25 except at stops then it is around 5-10 (maybe lower). I have replaced sender (electric) and even connected mechanical aftermarket gauge which gives me about the same readings. Pressure seems OK till engine gets to operating temperture, then as long as RPM's remain higher then idle everything is OK. My concern is the low pressure at idle which could be below 5 lbs. I am thinking oil pump.