Last month I was given an S40 to fix up in my spare time as it was burning oil and the previous owner couldn't be bothered to fix the issue. I went through the basic checks and eventually came to the conclusion it was the PCV's fault, specifically the oil separator, as oil was apparently pooling up in the inlet manifold. After locating the oil separator and removing the front, radiator and inlet manifold to get at it i found that oil residue was in the outlet of the oil separator and so as a temporary fix to get it through its MOT have put some carbon filter in the oil separator. The car now burns little to no oil however i now have an "engine performance reduced" light turned on due to what i can assume is the increased crankcase pressure. Seeing that the oil separator is not doing its job without an added filter, and to be honest i don't know how it was supposed to do its job in the first place seeing its essentially an empty piece of plastic, my question is:

Is there any easy way of making the oil separator actually separate oil from the air without adding something in the oil separator and thus increasing crankcase pressure?

Many thanks in advance