I'm not real familiar with diesels, so if someone more knowledgable can
dispute this, then take their advice over mine.

However, on my engine (as well as every engine I've ever owned) the
breather was a fairly cheap device.  I used an aftermarket one from Pep
Boys on my Blazer recently when I was replacing so much stuff under the
hood.  Mine was like $10.  It was chrome, so I used hi-temp black spray
paint for BBQ grills to "de-bling" it.

Concerning the blow-back through the thing...  Breathers as supposed to
relieve crankcase pressure.  If you're getting enough pressure to blow
oil through the thing it would seem to me that too much pressure is in
there in the first place.  That's usually indicitive of a bad ring
isn't it?

Either that, or the thing has soaked up so much oil after years of use
that it truly is clogged and now that stuff is seeping out.

But to answer your question, no, it's not adjustable.  At least not
that I've ever seen.

Regardless, it's a cheap part.  Replace it.

Just my $.02...deferring as always to someone more knowledgable.

~jp