After my truck warms up the oil pressure drops and the check gauge light illuminates. When the engine is cold at start up the pressure gauge reads 30PSI plus. After it warms up and at a stop in drive the pressure will drop to approx 5 PSI and will even dip to 1 or 2 PSI. Oil being used is 10W40. Mileage is approx 140 000 Idle is a little rough but not too bad. Engine does not burn oil. A little smoke on start up but not bad. I am thinking of replacing the oil pump, but will that be a waste of time/effort. Vehicle info: 1988 C1500 Automatic Transmission 5.7 liter 140 000 approx miles Thanks for any advice or assistance.