The 2013 Subie owners book says use 87 octane fuel in both the 4 and 6. Why is there a question about this?
Are you asking what to use? There is no question 87 is fine for those 4 cylinder and 6 cylinder Natural aspirated engines, they do have high static compression ratios, so high octane fuel is of NO benefit. Now an STI turbo does benefit from higher octane as the turbo really just boosts the cylinder filling and DOES increase the dynamic compression ratio. If your into power read about thermodynamics and increased compression ratios. Kinda easy to understand just a time investment to learn.
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