The octane selector is not used to set the timimg. It's original use was to tailor the timing to either higher or lower octane fuels for the best performance without pinging. You can't find fuel with low enough octane to ping in your truck; the engine will fight the starter before it will ping. Apparently the previous owner advanced the distributor timing too far and fine tuned it with the octane selector. The selector should be set to zero when you unclamp the distributor and time the engine. The "A" is for advance and the "R" is for retard. Sooty plugs indicate a rich condition or incomplete combustion, not an excessive advance condition.