Both X50 and X40 will remove O2 (DO) and CO2.  Removing dissolved O2 takes a stronger vacuum. If you are using a strong vacuum with X50 you will pull a greater amount of water vapor though the membrane. The thicker X40 membrane helps minimize the amount of vapor from coming through the membrane while still removing DO. The X50 membrane fiber has a thinner more porous wall for faster gas transfer than X40.  X50 is normally used for CO2 removal since the concentrations are often many times higher than DO.