The M54 engine has low tension piston rings, and once the rings start getting clogged with carbon buildup the engine will start using more & more oil (this might be one reason why some people mentioned after 70 k miles, the car started to use oil - my e39 did the same thing).
using the PCV will increase vacuum, and the apparently the scavenger rings somehow will perform better and hence no or very minimal oil consumption with this setup (CCV). However, it is unsure how long the engine will last with setup. Poolman on the e39 board had a catch can setup, and one piston had burnt valve(s) after driving a few years with no oil consumption. Not sure if it's related to higher vacuum.
However, on another e39 board, O2Pilot introduced another mod using the OE CCV system, by introducing another hose between the plugged off CCV nipple & plugged off intake manifold nipple (back of the engine).
This is the whole post, the fanatics guys always refer to (they use this mod), and post #74 has the resolution. The whole thread (looong) is worth a read.
Also, below is how the M54 pistons look once they start getting clogged with carbon buildup. the user Mlody from the fanatics has an outstanding post about this, and his battle to clean the rings in order to reduce oil consumption.
Here is his post
