How often is jet oil changed? Basically never. Air from the pressurized parts of the engine leaks into the oil system. In a way this is good, because it keeps the oil from leaking out into the airflow, which in older designs is used to pressurize the cabin. It's also bad because the air must be vented, and it unavoidably takes some oil with it.
Loss of oil during flight is rapid enough that the oil never gets old enough to need changing. It is topped up often.
