2000 Ford F-150 Oil Pressure Gauge

I have a Ford F150 with 90,000 miles. When I start the truck for the first time during the day and it has been cold that night or morning the oil pressure gauge reads that I have zero pressure then it will jump back to normal than back down to zero and this will continue for the first 5-10 minutes until the engine is warmed up. The engine does not act up when the pressure drops and it runs smooth. Also I notice when I rev the engine if the oil pressure is at zero it will jump back to normal? Whats the deal? Thanks!