outlets are 11mm ID IIRC, filter is just a K&N one.
i suspect the oil may be a little coked up though and overdue an oil change anyhow, maybe the burton cover needs baffles to stop oil surging into the breather holes or that the return to block isn't as smooth as it should be, as the pipe is kinked a bit in places.
just odd that it doesn't happen on boost, just with higher rpms it surges oil to the breather and it isn't coping separating the oil from the vapour too well, maybe i need a bigger breather pot, it isn't the largest by any means @2.5" diameter and 8" tall.