AFTER DRIVING AT ANY SPEED FOR A DISTANCE OF 1/2 MILE OR MORE THE OIL PRESSURE GUAGE DROPS TO ZERO WHILE TRUCK IS IN GEAR IF I PUT IT IN NEUTRAL IT GOES BACK UP TO NORMAL I CHANGED OIL AND FILTER AND FLUID LEVEL IS AT REQUIRED AMOUNT THERE IS NO CHANGE IN ENGINE SOUND WHEN THIS HAPPENS AS WOULD EXPECT IF NO OIL PRESSURE WAS ACTUALY HAPPENING COULD THIS BE OIL SENDING UNIT OR OIL PUMP GOING OUT