Last weekend the good folks at J&S Aviation installed an M20 air-oil separator for me. I hope to have some photos up here in a couple of days.
The separator is installed on the left side of the engine (looking from the front of the plane). It is clamped to the baffling next to #4 cylinder, and the oil return line drains into the magneto cover plate on the accessory case. We also rerouted the crankcase breather tube to connect to a stainless steel tube which is clamped to the exhausts on that side of the engine. This will result in any remaining emissions being vaporized on their way down the tube. Oil streaking on the lower cowling will be a thing of the past.
Since I have only 45 minutes of flight testing the separator, it is too early to determine what impact it will have on oil comsumption. Given that most of my oil usage seems to consist of unburnt oil leaving the sump by way of the crankcase breather, the impact ought to be significant. More reports as I accumulate data.