I'm conflicted. On the one hand I totally agree - if there were a major oiling problem like restriction or leak other stuff would be showing symptoms as well. But on the other if there were a manufacturing defect with the lifters (like in the thread others have referenced) what are the odds all 16 would be defective.Cool tar running up hill in the winter time flows faster then that. I still say it's the lifters themselves otherwise you'd burned up bearings down low.
Only one way to find out at this point.