at Idle the oil pressure gage will drop to 0 rev it up and back to normal ? is it a sending unit or gage or worse?

 

What year and engine and such for the truck? Always best to get that thing verified with a mechanical gauge when in doubt. Have you checked the oil level?

 

I would imagine you could disconnect the harness at the sensor itself, and then take it's ohm reading. More than likely you have a bad oil pressure sending unit. They break. More common than you think. I am pretty sure there is some type of rehostat that may have a break in it. Kinda like when your healight switch dimmer would break, and you only had full dash lights. You could not dim them at all. Same principal with the oil pressure sending unit.

Good luck.