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well, premium is hardly ever "mandatory" on an unmodified modern engine in the sense that you probably won't do any long term damage from using regular 87 AKI fuel. Subaru EJ255/257 turbo engines don't take kindly to regular gas. That being said, some GDI engines were specifically intended to run ok on regular fuel like the Ford Ecoboost.
Premium is recommended/required for the Toyota GR series GDI engines and I wouldn't be surprised if the FR-S/BRZ engine also requires it. You would probably be noticeably down on power and fuel economy if you didn't use it.
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