We do not have as much experience with horizontally-opposed engines as with V-type or inline configurations, but we expect all of our fuel additives to be fully-compatible with your Subaru boxer arrangement when used as directed. The only exception might be with the use of combustion chamber cleaners that are applied by manifold vacuum induction via the PCV valve or brake booster such as our B-12 Chemtool Total Combustion Cleaning Kit (part #2610). However, that is not as much a boxer factor as it is a vacuum system design consideration since some configurations only feed one particular cylinder. You might check some of the Subaru forums online to see if your concern is addressed. That said, we’ve talked to pilots who have used Berryman B-12 Chemtool Fuel System Cleaner (part #0116) in small aircraft that have horizontally-opposed engines and have only had positive results. If you wanted to be on the safe side and only use an additive a few times a year, we would recommend our B-12 Chemtool Total Fuel System Clean-Up (part #2616) about every 3,000 miles.