I will second strong recommendation against any extra complications. I will present a personal example. My other diesel sedan, I decided that in order to help to reduce amount of oil in my intercooler and intake, that I would install an Oil Catch Can. After all, there are some oil vapors that escape through the PCV valve, and there were plenty of articles on forums of people installing those. I went with Mishimoto Oil Catch Can - great build, fully metal, bronze washable filter. During summer months, I barely had anything collect. But once the weather god cooler, I had some nasty mixture collecting. I thought I was doing well. I washed the can and the filter every few months.
One afternoon, as I was leaving my work, I started the car. In my peripheral vision, I thought I saw a plume of smoke. I chuckled and presumed it was some annoying deleted vehicle that just passed behind me that left a lot of smoke. But as I got on the interstate, I started second guessing. I have OBD Bluetooth dongle, so I plugged it into the car and opened my dashboard. The car was showing EGTs in 600s. This is normal during a regen, but the system said regen was NOT active. I drove home, parked the car, and checked the oil level - MINIMUM! That means I was 1 quart below limit. Car has not complained yet, but I have never lost oil. Car had 51,000 miles - you guessed it, 1000 miles out of bumper to bumper warranty!
To make long story short, cold weather ended up causing oil vapors to cake on bronze filter and clogged it. That cause rise in crankcase pressures and caused turbo seals to start leaking. Solution was simple - reverse my catch can concoction. The benefit was not significant, and only seasonal here in Mississippi since it stays warm a lot of the year - and the risk was tremendous. I am happy to say that since reversing it, I have not seen much sludge on the intake valves, I have not seen any oil level dropping that is noticeable between 10k oil changes. And I learned my lesson - if it ain't broke, don't try to fix it!
Now I will argue that fuel-water separator could probably be useful for us, since HPFPs tend to fail on the Germans. And the main culprit has been hypothesized was related to water contamination of the fuel (and to a lesser extent gasoline misfueling, but not likely with anti-misfuel valves). Extra oil bypass filter === my question is WHY?