Jay,
You have the M-B bug!
Diesels have no PCV valve.
The lube oil in a diesel is typically black (soot in suspension), soon after an oil change. Black is OK - normal. But know that a diesel requires diesel grade lube oil that's designed to carry that soot in suspension.
Doubtful that cam box's oil drains to crankcase are clogged.
A loose exhaust valve guide would dump oil directly into the exhaust manifold. When the engine is run long enough to get hot that oil will begin to smoke and the car will lay down a real smokescreen!
There won't be any significant broken pieces from a valve guide - perhaps a few crumbs of metal but no chunks.
So, what to do next: Remove the valve cover or take it to a shop. The cover can always be replaced after a look about - so what if it leaks a bit until the engine is fixed. So the question is who will fix it if, in fact, it's a bad valve guide. The valve guide is supposed to be solid in the cylinder head; this one is suspected of being at least loose in its socket or (worse) its socket (now) being worn. The extent won't be known until its carefully examined. Realistically the cylinderhead ought to be removed, but if not too bad, an experienced shop may be able to fix it without removing the head - buy those guys a case of something good!
The exploded parts diagram shows this to be a beautifully simple engine. It has cylinder liners (steel sleeves that are fitted into the block that can be replaced when worn so the cylinders aren't bored out). One can buy almost everything as a part except the block! You'll have fun with the parts diagram.
Let us know how its going.
Tom