I would put what the manufacture recommends, they built the engine and know what it needs to be healthy. Octane is resistance to detonation. Higher compression needs higher octane because when you slam all the air and fuel into a smaller space (higher compression) it results in hotter temperatures. Gasoline will self detonate when it reaches a certain temperature based on the octane rating. If octane is too low you risk detonating and causing engine damage. So i would put 87 or 88 depenging on what you get where you live.
Where i live the pumps have 86, 88, and 90 or 91. so since 86 is lower than required i use the 88, no reason to go any higher as it will have zero effects on mpg or performance unless tuned for it. But definitely dont want to go lower than 87.