Yesterday morning when I cranked my truck to go to work my oil pressure didnt even come up, until I started driving down the road at 45 MPH or more then it went back up to normal. When I stop at a stop light my pressure drops very fast to 0 psi. I have been racking my brain as to what it is. If it was the oil pump I would think it would have no oil pressure at anytime. The only other thing I can think of is a spun main bering. Can anyone tell me some signs of this. My motor is a 390 and has about 70k miles on it. If it is a spun main is their anything I can do to get my oil pressure back up?