Pistons wear due to the fact that one side of the piston is the "thrust side" due to the
pressure excerted when the piston pushes the rod down on the power stroke. This wear
is diminished to nill, because the rod squirts a stream of fresh oil to the thrust side of
piston on every upstroke revolution.

This is related to full pressure lubrication systems, like in 4-cycle cars and motorcycles.

FBD.