Angier,
  I've had two different air-oil separators on my engine, an M-20 and a Walker, which is now marketed by Airwolf.  The M20 was a total waste of money.  The separated oil seemed to either stay in the separator or run out of the vent hose onto the belly while the engine was running, then invariably drained whatever was left over out after shut down onto my hangar floor.  I checked to make sure it was clean and had no blockage internally or in the lines and it just never worked worth a darn.
  I took it out, cleaned it up, sold it on eBay and used the money toward the more expensive Airwolf.  It's worked as advertised and has never dripped a drop.  My belly stays clean.  One caution about Airwolf though - it's advertised to come complete with everything you need from hoses to clamps and all hardware.  I gave them my engine model number and they sent me the hardware for a Saratoga installation, which didn't work in my ES.  I had to return some parts and then wait seven months for a new siphon tube for the oil return line.  It had to be fabricated to my specs because they didn't have one for an angle valve IO-540.  When I finally got it, I was charged over $100 for parts and shipping.  So much for a "complete" kit.  If you buy one, get Johnny at Airwolf to guarantee you that you won't be charged for any extra parts.  I felt I got burned by him on this.
  One thing I was told was that if your engine doesn't have oil on the belly, you don't need a separator.  I'd resisted buying the Walker for my plane due to it's large size and cost, but they now make a smaller diameter model that will fit much more easily.  (I only wish Johnny had told me it was coming out - I'd have waited for it).  Bottom line though is that the Airwolf is the best separator out there that I know of.  They're not cheap (around $600) but like anything else, you get what you pay for.
  Skip Slater
  N540ES