my oil pressure guage says between 0 and 15 lbs of pressure and does not move much between firing it up and running rpms. the oil is full, the truck seems to run fine and does not clatter or overheat. what is the deal ? whAT SHOULD I CHECK? IS THE OIL PUMP PUMPING, IS THE GAUGE SHOT, OR DO I NEED TO REPLACE THE SENSOR FOR THE GAUGE, AND IF SO WHERE IS IT?