are they definately plastic? could be bits of old rocker gasket? on one of my cars i went to take off the rocker cover, and the old gasket was still the original manufacturer one, 14 years of age, and was basically like fossilised wood rather than cork gasket! crumbled into a million little hard bits that i had to scrape out from around the rockers all through the inside, and after rinsing with engine flush for 15 mins, more of it came out the bottom, and had clogged the oil filter.
your blockage may well be the oil filter, when was it last changed?
if those bits are circulating through your engine oil system, they will get caught by the filter, and if there was enough of it, the oil filter's flow would be severely reduced