Originally Posted by PasserOGas

There IS NO WATER SEPARATOR. There is no dirty filter. They don't exist. What you describe is textbook fumes. It happens on descent due to valves opening to compensate for the reduction in thrust.

There was a video describing the reasons and times this was likely to happen (APU start, initial climb out, descent, almost never cruise), but now its locked behind the APA login page.

The most typical is a small puff of fumes on climb out, which passes and may or may not be noticable, then a major dose on descent as all the oil that has accumulated in the lines during the flight gets released by the wider open valves.