Where do you come up with one year versus 3-4 weeks of time between fillups? Vehicle tanks are vented thru a controlled system, not open venting. I store my boat over winter with premium (non-ethanol) over the winter with no loss of power.
Also stations get regular tanker fills so they have fresh fuel to blend the 93 to get 89 octane. Not months without delivery.
The PCM checks for knock and then adjusts to a lower table if needed.
on the 5.3 push rod 93 will not make much difference unless in AFM mode.. that is the only thing it will improve .
unless it's a gen5 with vvt or gen4 4.8l/5.3/6.2l with vvt
another thing is Many gas stations will scam you and give you 87 octane at the 91/93 pump
it happened at 6 gas stations i went to. AND they were name brand stations BP/arco and Shell stations...
the only way to verify it is checking the ignition timing. with the obd2 reader...when it happens .. That is how i figured out they scammed me...
91octane can get upto 42 degree advancement mid 30s on average....
AFM can run long periods at 12-16 degrees advancement.
93octane can get upto 42 degrees advancement high 30s on average
87 octane could barely do -12 to 12 degrees advancement
no AFM may cause knock or loop of death
i had fuel sitting in my car for a year when the tras went out it still started
used as the neighborhood bait car as there was a string of break ins caught them on the 3rd time (left the door open with the alarm on) they got too greedy.