My 2001 Subaru Legacy uses regular unleaded gasoline (87 Octane). Can my mileage be increased by using premium (93 octane) gasoline? If yes, by how much?
No. Not at all. The higher octane simply produces a slower burning fuel more compatible with high-compression engines. It has no extra energy or magical mileage powers.
No, it offers you no advantage at all unless your engine is designed to use it.
No benefit will be derived by using premium fuel in engines that only need regular. Higher octane fuels are needed in higher compression engines to operate properly, but in your case all that would happen is you waste money.
Octane rating is simply the fuels ability to resist detonation in high compression engines. Low compression engines don’t need it. It doesn’t make them any more powerful or efficient.