The oil that exits the engine and into the muffler gets bounced around a lot inside the muffler when it hits the cone or baffle plate. As it migrates back and forth inside the muffler it picks up any metal particles that developed from the loose muffler halves. The muffler pressure nipple connects directly to the inside of the muffler, allowing contaminated exhaust oil to re-enter the gas tank via the tank pressure line. Funny that most never realize that they are burning a higher oil ratio in the engine the farther they get into the tank volume.