Not using ARO isn't problem in my opinion, I don't use it myself and haven't had any problems.
Once a gallon may be stretching it for the filter, but it depends on your running conditions.
You say it was running fun the last time you checked, did it sit for while? It may just need some cleaning/tuning or new glow plug.
Haven't had a 3.3, but 3 gal may be all it was good for, engine life varies engine to engine. I'd try cleaning out the carb/fuel sytem and a new plug before going to the rebuild.