I've been playing with this for a while, but have yet to see anyone have a similar issue and I'm stumped as to what exactly could be causing it. In short, unless my ventilation setup is open fittings or open lines off of the valve cover it will spew oil out of one of the hoses.

My setup:
Rebuilt engine with roughly 5k miles on it currently
95 Block with Supertech 9.5:1 pistons, Wiseco rings, and Eagle rods, all ARP hardware and OEM seals
2003 Head with Volvo springs and Supertech undercut valves, OEM valve seals
Kraken GT2560 kit with MAC valve boost controller
MS3Pro - currently still on break-in tune making just south of 200hp
Valve cover modified for 2 -10 AN ports

Car has great compression and phenomenal leakdown results
I've tried a handful of ventilation setups and the only ones that seem to not spew oil on some way are those mentioned above, where there are no catch-cans, PCV valves, or anything to restrict flow, just open -10 AN ORB fittings off of the valve cover or two -10 AN lines that are left open on the opposite end. I've tried having a Radium PCV valve off of the valve cover with an open fitting on the other side and it filled a catch-can with two weeks of casual daily driving. I tried blocking off the intake-side port and running a single -10 AN open fitting and that covered the valve cover in oil. The most confusing iteration is the one pictured below though, the intake-side line is routed into a VTA catchcan and the exhaust-side is currently an open line until I get a second can to route in to, but as pictured the exhaust line puked oil despite not having anything that should restrict it the point of doing so. The catch can does neck down from -10 AN to M14 or something of that nature, but it's not a high horsepower car and still has another side venting to atmosphere. It's also worth noting that the amount of oil in the last picture is from a 10 minute drive after cleaning the engine bay of any oil I could find. I'm curious if anyone has ever seen or fixed something like this before.


Overview of the current setup, exhaust line is open -10 AN hose and intake line is ran into the can


The line off of the exhaust side is purposely led into a spot where I could see the oil gather if there was any, this amount of oil came from a ten minute drive.

 

Maybe I'm missing something, but isn't one of the fittings on the catch can an outlet? Can you take a photo of the side of the catch can that you have capped off in the photo above?

 

The line is routed into the ‘in’ side and the ‘out’ side is blocked off.

 

https://www.miataturbo.net/engine-pe...-thread-99549/

 

I'm not sure that breather vent is doing what you think. If you pull the hose going into the catch can off the valve cover and blow through it does it flow easily? My guess is that the catch can is effectively sealed right now. Something else to try would be sealing the top breather and putting the filter on the output of the catch can.

Or throw both of those hoses in some blender bottles with steel wool, drill some vent holes in the lids and go drive the car