My the manual wife's 2011 Imprezza with the 2.5L doesn't say anything about synthetic one way or another while the same 2.5L for the same year Forester says synthetic only. On the Frontier, my 2013 manual calls for oil changes at 3750 while newer Frontiers say 5000. Engine is the same 4.0L as far as I know. Just goes to show that manufacturers are inconsistent on the subject.I'd think those with a synthetic factory fill may have an even tighter tolerance to do so.
Maybe call an engine builder and get a current day expert opinion on the subject?