1998 Chevy Truck oil pressure

I have a 98 chey truck with 135,000 miles when i first start the truck up the oil pressure drops to 0 and then goes up to 40 wich is right at the middle of the gauge is that normal? also it kinda makes a light clicking sound at times just when i first start it up. and after a few minutes of the truck has sat in iddled the oil pressure neddle drops to between 0 and 40 but when i give it gas it jupms 40 and when i let off the gas it dropps back to 0 what does that mean