Hi Randy.....that's normal for all the castor that I have seen.
I have a couple of Bottles of Sig and Morgan's castor and they have the same crap in them.
It's dirt.
The oil is so thick that it would have to be heated quite a bit before that stuff could be filtered out.
What I have been doing is to not shake it up, just pour what you need and leave the dirt in there collected on the bottom.
I usually store the bottle tipped about 45 degrees so the stuff will collect in one spot....easier to see this way....HEH
I still find the same stuff in the engines though.....filtering the fuel in the airplane won't stop that stuff....it's too fine.....just avoid as much of it as you can.

Dave