1990 c1500 with 5000 miles on a new stock gm motor. Oil pressure has always fluctuated as normal but today it stuck at 15 psi. Once I start the truck it goes to 15psi and stays there even when I rev the engine. When I turn the engine off it goes back down to zero. Oil level is good, mobile 1 high mileage. I'm thinking it may be a problem with the gauge but is there something else I can switch out easy enough to troubleshoot this? Thanks!