Jet engines also have breathers. There is always gas leakage across the bearing sump seals that gets into the oil system and then gets returned to the oil tank with the scavenged oil. In fact engines employing labyrinth type seals require a controlled leakage across the seals to work. The seperator simply helps get the air out of the oil before the oil is pumped back into the bearings. The air is vented overboard. Some jet engines use a centrifugal seperator as described above however the simplest seperator can be a metal sheet in the top of the oil tank that the scavenge oil is made to flow over so the air bubbles can quickly seperate from the oil.