During construction I installed the homebuilder model Air-oil Separator from Aircraft Spruce. This unit has worked well so far keeping oil off the belly, but because of the way it's mounted on the firewall, there's no easy way to plumb the return oil line back to the engine. Instead, I have a plug in the return line and the oil that accumulates in the separator just stays put until I drain it during oil changes.
During the first 10 hours, I drained about 3 oz of oil from the separator. That's about half the capacity of the separator, and I'd like to reduce that if possible so that I don't need to drain the sump in between oil changes (every 25 hours).
I've read about other VW conversions using stainless steel pot scrubbers down inside the breather vent. The scrubbers increase the surface area exposed to the oil vapor and promote condensation back into the engine rather than passing through the vent tubing and overboard. Stainless Steel scrubbers can be purchased under the "Chore Boy" brand, and I found a 2-pack at Wal-Mart for about $3. One chore boy scrubber is just about perfect to fit into the cavity below the breather vent. Even when the scrubber is compressed to fit into the chamber, air still moves freely through the breather. One this I don't want to do is restrict crankcase breathing, as this could lead to oil getting pushed out past the prop hub.
After flying with the Chore Boy for 25 hours, I am very pleased with the results. Before installation, the air-oil separator would accumulate 2-4 oz at oil change time. Now, there is only about 1 tablespoon in the separator!
NOTE: photos link to full size image