Engine Driven On Board Air from Kilby Enterprises
The OBA compressor will put small amounts of oil in the air. You should have a coalescing filter as the first component in the plumbing. But, not too close to the compressor. Use at least 4 feet of air hose between the compressor and the filter.
Kilby provides a smaller diameter hose to connect the drain side (bottom) of the filter to the compressor. He also provides a manual valve to keep the drain closed until your filter is full.
I mounted the filter on the firewall. Kilby provides a nice bracket. I used about 5 feet of air hose and cable tied it to the engine compartment strut, which is now back in place.