Pre filter? You mean the PCV Crank Case breather filter? If so, the old cottom type must be replaced at every inspection (as you say, never gets done) and this blocks and causes crankcase pressure and eventually turbo failure. However, BMW have re-designed the little filter and made a new one that is lifetime service free and works on a vortex type system that uses airflow to seperate the oil in the breather system instead of the cotton filter - Only about £35 and well worth changing!