The correct answer is D) the system has been drained and then filled with new oil. Purging air from a hydraulic system is necessary when the system has been drained and then refilled with new oil. This is because draining the system introduces air into the lines and components, which can cause issues like erratic operation, cavitation, and potential damage to the system. Purging the air ensures that the system is properly primed and operating as intended. The other options are incorrect because they do not necessarily require air purging. Option A is incorrect as an idle system may not have introduced air. Option B is incorrect as adding small amounts of oil should not require air purging. Option C is incorrect as overheating the system, while potentially introducing air, does not definitively require air purging.