GMC Repair: My oil pressure on my gauge reads 18 to 20 PSI at IDLE, oil pressure gauge, oil gauge

Question
When I start my GMC jimmy up in the morning, it will idle at 42 psi or so and even after I get going. After it has been running for about 20 mints to 30 mins, the gauge will start to drop as i am not stepping on the gas pedal and as i come to a stop or just about to idle the oil pressure gauge will be about 18 to 20 psi. It has not gone any lower that I have notice. I have checked the oil several times and it is right at the line of being full I will step on the gas pedal and the oil gauge will go back up to about 40 to 42 PSI. Is there anything to be concern about or is this normal do you think. My vechicle has about 138,000 and it is a 1999 GMC jimmy. Thanks

Answer
i personally own a 1999 Jimmy with over 180,000 and that is right where my oil pressure is at when idling, should not be a problem so long as you have oil pressure, only fix i know of would be a new oil pump but its not really affecting anything by running at around 20 when idling