My thinking is that if it's PROPERLY designed, an oil separator can be of benefit especially if you have an aftermarket tune or if you're using less than premium fuel since besides helping to keep the backsides of the intake valves clean it may help to prevent detonation. In my view a properly designed separator will remove the maximum amount of oil from the air stream while still allowing full unrestricted evacuation of the crankcase pressure. I haven't personally examined every brand of separator on the market, but I have reservations about some designs I've seen. One in particular concerns me. It used a felt like material in addition to a steel mesh and screens. I don't doubt that the felt would be effective in trapping oil, but once it becomes saturated I think it is highly likely that it would seriously restrict the proper evacuation of the crankcase. If that happens the best you can hope for is that the crankcase pressure will force oil up and out the dipstick tube and make an oily mess on the driver's side of the engine bay. At worst the pressure will find a way out either from valve covers, timing cover or front and or rear main seals. I use a Bob's separator and IMO it is very well made and designed. The drain valve make for easy service too.