87 or 94 octane

Check the cylinder pressure.
anything less than 150 should be able to run 87.
Slantys usually run 145 or less so, they'll run on almost anything, especially in a lightweight car.
By relating it to pressure, this automatically compensates for altitude changes.
If you have a tired old oil-burning clunker, then that oil in the chambers, could initiate pre-ignition, leading to detonation, and requiring more anti-knock in your gas. Don't confuse this type of situation.