Friction Modifiers are plastics and accumulate if not flushed and replaced. That is why when you lift the valley cover/manifold or sump it can look like a dry muddy riverbed (cracked mud).
We used to call it the Castrol drought. Horrendous product and don't use Castrol to this day because of it.

Shell took over Pennzoil some time back and that is why we don't see Pennzoil in Australia like we once did.