The A85 from Tyndrum to Dalmally is a very nice run, lovely corners and changes of gradient. From there to Oban the road is nice but nowt special. The run from Callander will take you up Loch Lubnaig, and on to Tyndrum, that too is a nice run. A detour, but its worth it is to head to Helensburgh, then onto Aberfoyle, then over the dukes pass to Kilmahog, (a few miles from Callander) and from there continue your journey. Another detour, is to head up to Inverarry, over the rest and bethankful pass. You take the A82 to Arrochar, but then stick on the A83 to Inverarry and turn off and head over another lovely road to the A85.
The road you mention runs down Long Long, from Arrochar (A814) and ends up in Garelochhead. Its a nice road when nothing is coming at you, but can make little people in the back of your car feel unwell. There will have been an accident on the A82, and they diverted you down there. The a82 from Luss to Tarbert has a very high accident rate for reasons unbeknown to me as its an easy stretch of road IMHO.
Be careful if your driving up here in Summer. We have a lot of foreign tourists that can, and do get confused with driving on the Left. They can, and do turn in front of you without looking. Bikers and caraveners also add to the blood bath when they meet (narrow stretches of the A82 seem a favourite meeting point)