Follow Jerry's advice. I would not drive the truck until I was sure of the oil pressure. Oil acts as a coolant and friction reducer. without out it your engine will soon be toast. On a warm engine pressure should read about 15+ psi at idle, 30-40 psi at speed.

It sounds like you might have two or more problems. Oil pressure or gauge, cooling system and a rusty gas tank. You need to take care of them in that order.

Fred
52 3600
69 C-10