[SpudRider]

Even if the OCS is upside down, you might not collect any fuel in the down tube. The fuel vapors are still condensing into the tube, but the tube might not be thoroughly sealed. If you shave/sand any imperfections in the plug, and seal the plug with grease, you will definitely collect a mixture of water and fuel in the down tube.

When the OCS is oriented properly, you will collect less condensed fuel in the down tube. This will better protect the environment, and reduce maintenance, since it allows you to empty the tube less frequently. It will also improve fuel mileage, but the improvement will be so small it will probably not be noticeable.