I would be very concerned if they are the pieces of the valve seals-which it sounds like they are. Check the pickup screen for your oil pump-especially in the hidden half under metal cover-theres likely to be more in there. A small piece entering your oil pump can cause it to fail quickly-seen it happen on many a tired 289/302- either pump locking up and twisting oil pump shaft off or stripping it.What can happen is when screen gets clogged, there is a hole through mesh in pickup center-normally closed by flexable metal plate, when screen clogs-it opens up and everything goes into pump unblocked-not good. Not knowing your engine and wether or not top half or seals have been done- I would check also. The early timing chains were plastic coated on gears-it is more of a whitish yellow color, but sitting in bottom of pain may darken.
Going to go with general consenise that it is hardened valve seals. The oil pick up did have a good amount of debris inbetween metal cover plate and screen. $9.00 part at NPD so i replaced it. did not want any pieces stuck there getting loose and possibly entering into engine.Don't know history on top half of engine, how extensive of a job is doing valve seals? ::
It isnt the hardened valve seats, those are metalic. It is the valve stem seals. the are the umbrella seals that go over the valve stem under the springs. I have had the same thing happen. the only realy worry is that the oil pump passage wayor a galley get plugged up, the rubber will not cause any damage itself, only secondary from plugged oil ways.
You can do the valve seals on the car, by using air in the cylinders and doing one at a time-old mechanics method. I would recommend pulling the heads-chances are if the seals were that bad, a valve job/guide work is also probably inorder. You could pull a valve cover and check to see if maybe this has been done before and the pieces you are finding are the remnants of the old seals. The seals themselves as mentioned are the mushroom type black rubber, you can see them by looking through the valve springs, using a small punch or something similar you can also test their flexability- with age they become very hard, which leeds to them cracking and breaking up and ending up where you found them.If they haven't been changed you will probably also find pieces laying up top as well,and missing/and or cracked ones/pieces lying inside spring. Sounds like you have already noticed the smoking, so your probably going to need some seals/valve work.
Do NOT go cheap on an overhead valve spring compressor. Ask me how I know. I saw one that looked decent at Napa. The one I bought from Advance was the biggest piece of junk I've ever had the displeasure of owning. It went in the garbage as soon as I FINALLY got the valve seals replace. I think it was made by Ampro or something.