Ranger Oil Pressure

I bought a 1988 Ranger a couple of years ago. It as a V6 4WD with 135,000 miles. The problem is with the oil pressure. At normal operating speeds the oil pressure stays in the normal range. When the motor is at operating temp and I stop at a red light that lasts a long time, the oil pressure drops into the low (red) zone. Once i speed back up the oil pressure goes back to normal. Will I harm the engine by letting it idle a minute or two at low oil pressure? I have been putting the truck in neutral and idling the engine up to maintain oil pressure, but that is an aggravation. I don't want to spend a bunch of money on the truck. It runs great, but the body is rough. I don't want spend $400.00 on a $800.00 truck if I can help it. Thanks. Danny.