A319,320,321 oil fumes in cabin
Engine oils should be topped up within 2 hours of shutdown. after 2 hours the oil tends to hide so over servicing happens on start this excess oil pumps out of the breather system.
During APU shutdown one should allow the FADEC to shutdown before turning the batteries off this ensures that the air intake door is fully closed thus preventing oil bleed back into the intake,ground crew should wipe any oil from the APU intake door and surrounds.
Switching batteries off too soon will also give an indication on the APU oil sight glass of low oil quantity do not service with oil unless you are sure the air inlet door is fully closed,over hastie pilots not waiting for FADEC shutdown are causing this aerotoxic problem.