1991 f150 xlt lariat 4x4 302 AOD. 95 percent of the time if the engine has been running for 30 min or more, if I stop and idle the oil pressure will significantly drop off. Once I continue after the stop the oil pressure raises back up to normal. The other 5 percent of the time the oil pressure stays where it should. I changed to a straight 30 weight but no change in behavior. I'm told there is an oil pressure regulator in the oil pump???