From my experience the Nismo oil air separator don't work that will in practice. I've been chasing oil issues on a 450rwkw rb26 for about 12 months now, we found that oil was pushed from the sump through the Nismo separator back into the catch cans (this was on the dyno), i've put a 1 way valve on it now
Since then i've plumed the dip stick to the cam cover to bleed pressure out of the sump, and even though it's only going through a 1/4" BSP fitting, it made a big difference.
catch cans are a band aid solution, the core of the problem is pressure in the sump.