The oil separator box is also refered to as the oil trap and PCV breather box. Over time sludge will tend to build up inside the box, especially on engines that don't get regular oil changes, and restricts the engines ability to breath. This will then lead to oil leaks from whatever source the air pressure can find to vent through.
If you are fighting oil leaks and you've changed the flame trap, your oil separator box may be clogged up.
Replaces Volvo 3501160