Pieces of steel in cylinder??

Right.... not as much. FWIW.... Typical levels used to be 1200-1400 PPM. SM, and now SN, grade oils have had the ZDDP level reduced to 800-900 PPM in the lighter weights, or about a 30-35% drop. That drop put high contact pressure flat tappets at high risk. Low contact pressure ones (like with stock cams and springs) seem to survive fairly well still.

And the reductions were actually mandated to reduce the phosporous to protect O2 sensors... the 'P' in ZDDP. The Z had to drop along with the P; there is a fixed relationship between the 2 elements.