I am having trouble with my mustangs oil pressure. When I first start the car up after not running it a day or two the oil pressure reads about 40 psi at idle and while cruising it goes down to about 30 to 35psi. But after a few minutes of driving it drops to less than 10 psi at idle and it stays at around 10 to 15 psi cruising at 60mph. And at idle when I rev the engine I see the gauge starts climbing. Please help.