I was using a bit of oil so decided it was time to look for solutions [motor is still healthy], so I'm pretty sure its nothing to do with that.
I took the PCV valve out and sure enough I could blow easily through both ends. I squirted some brake clean in it a few times and some black crud came out, blew it clean, squirted with a small bit of silicon spray, and it seemed to 'fix' the problem. But I can still blow a tiny bit back, so I don't have a perfect seal. Blowing forward is no problem = full flow.
I reinstalled [its not a big job to remove or replace], I will likely order a new one anyway, but was just wondering how good do they actually seal?
My question is should you be able to blow back through the valve at all?