I bought a 1984 truck with a 75000 mile 360 on it from a local dealer. I talked to the previous owner who was a Penzoil faithful and told me how he changed the oil every 3000 miles. The dealer changed the oil before I bought the truck. It lasted about 1000 miles before the detergent motor oil they put in dissolved all the Penzoil buildup and stopped up the pickup tube. I was 100 miles away buying the Cummins diesel to go in the truck when my oil pressure started dropping and my lifters started clicking. I barely made it home. It maintained around 10 psi if I kept it below 50. It shuttered and seized up when I pulled into the driveway at home. I still have the motor. It looks like someone filled the valve cover with black cornflakes!!! Check the filter first and then check the pickup. My first wife's mini van did the same thing when I put in some Lucas oil treatment. It dissolved the Gastrol buildup in little flakes that clogged the pickup tube. It lost down to 10 psi and the lifters really made a racket. I dropped the pan and cleaned out the pickup. I dropped a bearing and it was fine so I cleaned her up and all was well.