I have a 1996 Chevy Silverado Extended Cab 2-wheel drive 5.7L. When driving down the road my oil pressure gauge is at 40. However, when I come to a stop and am at idle speeds my pressure drops to 20. Up until recently, even at idle speeds, the pressure has stayed at 40. All of a sudden my pressure has begun to drop down to 20 at idling at a stop. Is this normal or is this of a oil pump going out or another problem? I've never encountered this problem so I'm not familiar. Please, any advice is helpful. Thanks. I currently have 182,000 miles on the motor.