I have no idea. Out of the four or five engine options I'm considering, the majority of them will need a return line. So, after thinking about it, I thought now would be a better time to open the tanks again, and get it done, rather than waiting until the wings are on, and then ordering the engine only to realize that I need the return line.
Some FI engine's don't need it, most do. The Subaru's for sure need it, and I'm sure the deisels will (as well as the Innodyn

).