In all likelihood, the oil separator is effect, not cause.

The only way to know, is to do a compression check and leak down test. During the leakdown, you can listen for escaping air. If it comes from crankcase, it's rings/piston/cylinder related, if from exhaust, it's exhaust valve and if from intake manifold an intake valve. If you have good compression and good leakdown, it is from valve guides.

Good luck,