I use a piece of tape over the muzzle for FRP. FRP is caused by the burning oxygen inside the can - once that oxygen is consumed, the pop is less noticeable. Tape, or a condom, even a cottonball in dry conditions, over or inside the muzzle of the weapon helps that FRP by burning more of the oxygen inside the can before the gasses are expelled. More shots fired equals less available oxygen to be consumed and burned.
Wet lubes inside a suppressor can make it collect more gunk, albeit that it does reduce decibels it will require more cleaning, later. On some proprietary integral rimfire suppressors (Ciener) it can make cleaning an impossible nightmare; they are very quiet, but fire them dry only, please.
An MP5 suppressor that cannot function wet is a mind boggle. I can imagine the troops involved in that fiasco were perplexed (to say the least) that their weapons should not introduce H2O.