Is it common for the oil pressure to drop when stopped or idling?

It doesn't drop to zero. It has a 4.3 160xxx miles and is 4x4. I am looking to buy this truck off craigslist, I dont have a vehicle right now and I need one soon. I found this for a good price, ran magnificently, transmission seemed to shift fine and seemed well taken care of. The only issue I seemed to notice is after I finished the test drive and stopped it to check all fluids warm and examine while running after driven is 1- the oil level was high and 2- after a moment I noticed that the oil pressure had dropped some, not really low but enough for me to notice.

The owner said that that is a common thing and nothing to worry about. I looked online and some people in different forums are saying that as rpms drop so does oil pressure, and some are suggesting more serious issues. The owner had recently changed the oil, the oil looked and smelled great, other than the pressure drop I failed to notice anything possibly wrong.