A crankcase breather is a bad idea under normal driving conditions. When not under boost the blowby and other vapors in the crankcase are removed by the PCV system. It is a closed system. When you put on a breather you are opening that system and depending on how you reroute the PCV system either are introducing a vacuum leak into your intake (through the breather and the crankcase.) Not good. Or you are completely disabling the PCV system which will allow those caustic vapors to accumulate in the oil. Also not good.