Can anyone actually explain how any ambient pressure contaminant can get into otherwise sealed, pressurised systems?
I never did quite understand what this question was about, so my answer may be missing the mark.
For starters the oil system is not sealed from outside air. It does have a scavenge just like your car engine. It does rely on knife edge seals as well as friction carbon seals. It does not only provide bearing lubrication but also adjusts the thrust load on the bearings via air pressure internally to the gas path.
For the thrust load balance as well as cooling of hot metal parts behind the combustor, pressurized air is drawn off the gas path (typically in the high pressure compressor rotor). This air can entrain dirt and this dirt can flow through the labryinth seals between the oil system and the internal pressurize cavity of the rotors. Note you want any air flow to come out the breather rather than oil flow inside a hot rotor compartment.