Hi Ian. I had this issue with my 95. Started around 180k, was still there at 289k. I replaced main and rod bearings, measured, ok gap. Replaced oil pump, same issue. Ended-up just running 50wt in it. My best guess for mine was that a cam bearing, or other tolerance had opened-up. Lots of them do this. But I have never heard of the cause. The 3.9 is just a V8 magnum with the front two cylinders shaved-off. I'm pretty sure a few of the regulars around here know the cause and may chime-in when they see this.