Factory oil separators are basically oil catch-cans. They started implementing these on GDI engines to reduce the amount of carbon build-up on the intake valves. I don't think it's necessary to install one, and it may be illegal in some states (you would expect them to be legal but our emissions laws are getting way too unreasonable). In my opinion, GDI engines are bound to carbon build-up whether you have a factory oil separator or oil catch-can; they reduce it but do not eliminate it completely. As a GDI owner myself, I always check my intake manifold and intake valves every once in a while to monitor the build-up. When it starts to get severe, then I'll do a walnut blasting on it. GDI engines should have a carbon cleaning done every 60k-80k miles.