[1969 GMC]

on my oil press. gauge, what should it be reading for an engine to be healthy? I know it doesn't have numbers, just the 4 squares. I thought my truck registered higher before, now it drops just below 1/2 when idling and is about 3/4 or more when I'm driving it . It seems to move along with how many rpm's I am running. I checked my oil level and its ok. What else could be the culprit? thanks