The owners manual for my 1934 Allis Challmers WC says 7psi oil pressure is normal. different tractor, i know, just thought it was interesting to note this.
I agree with the other comments given so far.
In an engine, a drop in oil pressure at low rpm indicates worn parts [relief valve, cam bearings, main bearings, rod bearings].
The clearance fit of all these parts is what provides the "back pressure" that gives you the oil pressure shown on the gauge (as long as the pump is working correctly). As these parts wear more and more, oil pressure will continue to drop more and more at idle, eventually it will drop at higher rpm also.