I had the same problem a few years back and it was the oil pressure sender. It was easy and inexpensive to replace. Deanyel is right about the harness to the oil pressure sender...the insulation was flaking and crumbling...about 2 inches of the wire leading up to the sender crumbled as I took it off. Used electrical tape and silicon spray to "fix" the harness. It's been about three years and it's still ok.