How pliable are the pieces? Can you cut a couple apart to see what they are made of? Could be the edges of an intake gasket that broke off and went through the
oil passages into the pan, or RTV that was used for sealing the intake, timing chain, valve cover, or
oil pan gaskets. If it is that pliable, I doubt it could do any real damage to the engine unless it blocked some
oil passages and reduced oil flow.
From the size of the pieces it seems to be something fairly substantial if they aren't soft- Piston skirts or babbit material are all that come to mind at the moment. If you have a torque wrench, taking the caps off one at a time and replacing them as suggested is pretty easy to do and will solve much of the mystery, IMHO.