I have a 2001 chevy silverado 4.8L v8 with 128k miles. Ive had the pan dropped and cleaned out and the engine flushed and keep the oil changed on a more than regular basis, the truck smokes a little when i first start it but it clears after a couple minutes. For the first 15 to 20 minutes everything is ok but after the truck gets hot the oil pressure drops to 20lbs. also there has been a knock in the motor for the last 40k miles. Id appreciate any help or advice.