The oil rings are stuck and blocked, it becomes an oil burner. The next step is they do not check the oil and keep it topped off so they run low on oil causing bearing damage.

They hear a knock noise and top it off. The knock noise is a rod bearing shredding it self and often times turning in the big end. (if not then it will do it soon after)
The final stage is the turning rod bearing seizes and snaps the connecting rod, often times making a hole on the block.

That cleaned engine a few posts back is a candidate for an oil burner.
I would use Mobil 1 and change it every 3k. in about 10-15K miles the rings will be free and whistle clean.