Engine question for the penny pinchers.

I'll give you the background first then the question.
My father bought a 90 Dodge D350 Pick up on Ebay tha was local to me ( He lives 500 miles from me). I didn't know about the purchase till it was made so I couldn't go check it out first, and asked me to go pick it up, told me to go ahead and register and tag it and he would pick it up some time in the future. No Problem I got it home and occasionally drive it a little and use it to pull a trailer from time to time. The truck is a 360 auto 4 speed auto trans with about 120K miles on it 100% stock. after probably 150 miles I have driven, I started noticing that when you start the truck up there is a second or two of slight bottom end rattle, and while driving all is quiet while accelerating or slowing down but while driving with the throttle in that neutral spot not speeding up or slowing down there is a very light rattle from the bottom end of the engine. another observation is the oil pressure gauge will read in the 3 to 5 PSI range at idle when the engine is at running temp. Now for the question
Am I looking at a worn out engine, a worn out oil pump, or do I need to run Some thicker oil? I do have two low mile '89 360's at my fathers ( 500 miles away) and I have built several engines and have no problem doing it I'm just trying to same a little cash on this one if it's possible. David