I know this doesn't apply to all vehicles but when you fill up with about 5 gal or more, the ECM knows this and reverts back to a high-octane table stored in the ECM. Your ECM has a memory of driving conditions...hot/cold, low/high throttle setting, etc, etc. The ECM will try to keep as much ignition advance as possible without detonation, if you have a low octane in the tank, it will run on a stored "low octane" map to keep the parameters at a narrower value.
When you have high octane in the tank, it will run on a different table to give you more spark advance thus..on certain occasions ie, full throttle, climbing grades, when you have more demand on the engine it will run a little bit better.
This is debated in depth so please don't turn this into an octane war
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When you fill up, the ECM goes for the higher map if in fact you used a higher octane. When you induce a load over a set number of attempts and it starts to ping, the ECM will default back to the low map and thus you won't get the max spark advance as you could since that is mainly why it's pinging.
As the motor spins faster, you need more advance to catch the piston in the right spot to maximize the fuel burn, however, at high rpm's if you fire the plug to soon, the rapid burning characteristics of low octane will reach peak pressure before the piston reaches TDC thus trying to force it back down the opposite way (upstroke/compression).
Think of a tether ball being hit in the direction it's already traveling...you don't get much impact, but if you catch it early enough and can get a solid hit, it will go faster..wait too long and you just catch the tail end of it.
Same with spark advance, retarded timing will give you a burn while the piston is starting to go to the downstroke and you cannot harness all the energy, more advance and it will be just right but the limits of low octane won't allow alot of advance..higher octane burns slower allowing more advance because the piston will clear TDC before peak pressures happen.
That is probably why the motor pinged after fill-up, you are now running higher advance on low octane fuel.
I really hope all that's correct
Sorry to be long-winded....please correct any errors, i'm always open to criticism.