On mid grade understand the tanker truck does not come with three grades of fuel. It comes with two and the driver mixes the mid grade at the station. THis leaves some room for error. If the driver mixes right then no problem.
The only grades your guaranteed to be absolutly correct is regular and premium.
Friend works at a tank farm and told me that's how it's done.
Octane is the burn rate/ flash point. Basicly the higher the octane the SLOWER it flashes. This is why it helps with ping particulary in a higher compresion engine. It effectivly slows the burn and there for corrects for a lean mix or too far advanced timming. That keeps it from igniting too fast and causing the ping.
Here is a much better definition.
Basicly you could tun the engine to run fine on regular and get close to the same performance. My take on it is the timing is advanced some. Old hot rodders trick ad yes if you bump the timing and run a higer octane it does work and make more power. Possibly inacurate timing marks on the engine and even though you did set the timing to factory spec the timing is actually advanced. It is not uncommon for GM harmonic ballancers to have the outer ring slip and it usualy slips to indicate lower then actual timming.
GM distributors are also prone to the mechanical advance hanging up (FI equiped vehicles do all the timing with the computer so no worry here). The distributor has a shaft within the main shaft. Vapors from the engine wil get between the two and coke up. This will prevent the mechanical advance from retading after being run at a higher RPM and it will cause a ping problem. Easy to correct if your not affraid to pull the distributor.
Another thing that does happen is carbon forms on the intake valves and pistons. When it forms on the intake valves it acts like a sponge as well as fouls up the air flow into the cylinder. Both will cause a lean problem. this can also cause a little bit of a run on problem because a lean mix has a lower flash point.
When carbon forms on the piston it can have hot spots that will ignite the fuel without spark. This is the usualy cause of run on especialy when coupled with a carboned up valve.
"Top end engine cleaner" from your local GM dealer. GREAT stuff and does clean carbon off valves and pistons quite well.