The reason you run higher octane when you advance your timing is because the higher the octane of the fuel the more heat energy is needed to ignite it. So you run less of a chance of pre igniting the mixture (detonation) with your timing advanced.

If your car is completely stock just run 87 octane. The car will burn the fuel more effciently then 91 or 93.

On OBD-II vehicles the timing is completely controlled by the ECU. It advances and retards the ignition timing based on many different factors and input from the sensor to maintain economey and performance when needed.

On OBD-1 you need to jump the ECU connector, most likely disconnect all vacum lines to maintain idle, then flash your timing light and move the distributor in the correct direction to either advance or retard your timing. When you are done tighten down the distributor reconnect the vacum lines and remove the jump and there you've just adjusted your ignition timing.

I strongly recommend you leave it within specs unless you know what you are doing.