What I think not everybody understands about multi-grade is that warm 50wt will flow easier than cold 15wt. But cold 15wt flows easier than cold 50wt, the same is true when theyre warm. You need enough film strength and clearance to prevent metal to metal contact. If parts flex even more clearance is required. The thinner the oil the higher the flow at a given clearance and temperature. More flow equals better cooling of the part. Higher temp equally thinner oil, too thin or hot and you lose load capacity. It's all a balancing act. My theory in this case is either the clearance tightened up or we had lack of flow, providing nothing broke at the onset. The temp and film strength would be the same for the other 7 rods. Maybe after teardown we'll find a obstructed rod feed galley, other rods that are out of round or a spun #5 main? I still like thinner oil and more clearance, but doubt if 50wt or if it had actual .002" clearance caused this failure. My 2 cents
Doug