Watch this video - it shows everything you need to know.
https://youtu.be/-LXBlUlgTjo
There are two types of oil separator - the filter (loo roll) type and the vortex type. The filter type possibly does a better job but can clog quickly so the the vortex type is more commonly used to reduce maint.
Filter type...
Vortex type...
Mine - old and new...
