This is my preferred route between Connel and Oban. There is a short stretch on the A828 and A85 in Connel but that's all in the 30 limit zone so not too threatening (and there's a footpath alternative - there are not many pedestrians). The "Old Road" that leaves the A85 at the south edge of Connel is a bit rough and muddy in parts; there are 3 gates and a cattle grid too. But absolutely no traffic apart from occasional horses and dog walkers. Bliss. From Glencruitten into Oban centre is busy but I don't know an alternative.
I find this better than the NCN78 along the "Back Road" or the stretch of A85 between Connel and Dunbeg (formerly the NCN 78 way).