Okay.... here's some info... the pcv system is the positive crank case ventalation system... this system pulls the "harmful" fumes from the crank case and pulls them into your intake track where they can be burnt in the combustion chamber. The pcv valve makes sure that this system only operates at times that would be desired based on throttle position idle and stuff like that. SO the problem is that when the intake sucks in these blow by gasses from the pcv system that they often have small oil droplets floating along with them. We know that oil doesn't burn well and that it will make a mess of your throttle plate and intake manifold so nissan aks "gee wiz engineers how can we get this oil out of the blow by gasses?" these guys use an oil seperator that lets the oil collect in this black box and then drain back to the oil pan. You want the oil to return to the pan or else you will be loosing oil. And here is the problem. For some reason the engine was designed to seperate oil only out of half of the blow by gas. In order to "upgrade" your pcv system you will need to reroute your hoses for 20 bucks or so... you can read all about it in this post...
http://www.sr20forum.com/showthread....highlight=blow
so that you know an oil catch can is slightly different than an oil seperator because the catch can catches the oil and does not let it drain back to the oil pan. Instead you must monitor the level of the catch can every so often so that it doesn't fill (which would take a while) which is why i decided to improve on nissans oil seperator so that oil would automatically drain back to the oil pan (even though some will suggest that the oem seperator is inedequate.)
So to answer your question the oil seperator removes oil from the blow by gas and should be used however, it will be much more effective if you reroute it as is discussed in the post above.
