Mrcrowley Yes they can stop up.
This is why they (EPA and others) removed a lot of zinc from oils. It would do this.
I dont have to run cats here where I live, so its just a header, resonator, and muffler and diesel oil for me.
Back in the day, my cousin had a Mazda and he was rebuilding and saw that on his pre cat.
It had stopped up and burned a valve he said.
So I took pre cat off (thats right below the exhaust manifold) and hulled it out.
It was not clogged at all, but I did it anyways.
