Higher octane requirements usually are for a reason. These days cars can pretty much deal with lower octane fuel (octane being anti-knock and not a measure of energy or BTUs BTW) using knock sensors and reducing boost / retarding spark/ignition, but there’s a cost.
That cost is often very minor, … the fuel mileage and emissions numbers that are published are based on the fuel that is recommended/required, and either (or both) could be worse with lower octane fuel. It probably has no negative effect on a modern engine’s longevity.
In my gasoline turbo cars I have been able to document (40k miles/year) about a 20% highway mileage gain by using the (recommended) higher-octane fuel, … which pretty much covers the difference in price, so in a way the extra fuel range and extra power is free. The engine is able to run a more advanced ignition and slightly higher boost, both adding efficiency and burning cleaner. For me the big gain is stopping 20% less often for fuel. Premium fuel also usually has a better additive package which could mean a cleaner and better running engine.
Diesels are a different beast, I live where cold weather brings winterized diesel, … essentially cut more with kerosene to lower its wax-point with a corresponding reduction in cetane and therefore lower power and mileage, I can lose close to 20% in fuel mileage on winter diesel.
