Modern ECU, fuel injected motors will automatically adjust to burn lower grades of fuel. You just won't get the performance numbers advertised in their brochures. I know for a few model years; VW's had an octane sensor that would trip the CEL if you didn't fill certain model cars with premium fuel.
My Prius was set up to run on 87, my ZR1 was set up to run on 93 "minimum", but I run 104 when I really want to play. I don't need "anyone" to tell me what I need...or want.
Considered this prior to using 89 oct in the 5G -- after all, it's designed to run on 87. 'Run' and 'get best mpg for my driving conditions', aren't and will never be the same tho. As above, modern ECUs have been using lambda to adjust to octane at least 30y now (those vehicles with wideband sensors as current state-of-art, will be much more accurate as well). No debate tho, 5G gets better mileage on 89. Have tried both a few tanks, proof enough for me. But 93 in a Prius? Spend the savings on an electric bike, for the < 3-mi honey-do runs that can ruin a Prius' ICE. Of note: since using 89 in the Prius exclusively, have not seen too much of that exhaust-blowby-caused, highly-acidic water accumulation in the pan on swaps. Could be due to swapping every 3500 mi rather than the manual's reccies (which AMD from Car Care Nut YT channel's repeated said, is the source of those Toyota which never make it even close to 100K). Seven oil / filter changes in 70K mi, are you f***ing insane? Just the filter itself would be fully-bypassing long before 10K, pumping unfiltered oil through the myriad tiny passages depending on crystal-clean oil in them, in modern ICEs. Change your oil -- esp in an HEV which rarely makes it to the same op temps an ICE-only burns off water in pan with all that wasted consumption, easily.
Yeah, but you shouldn't encourage it. Priuses use a 2.0-liter 4-cylinder Atkinson-cycle gasoline engine. and thus 87-octane RUL is highly recommended. In these engines...."the intake valves remain open longer to expel part of the air-fuel mixture back into the intake manifold." This means the compression ratio is effectively lowered in these engines instead of the otha way round! The term "premium" is a marketing label. ...."Yet demand for premium gas is up. That makes some sense: More new vehicles recommend or require premium-grade octane levels of at least 91. But many drivers falsely believe high-octane gas offers a kind of treat for their engines." True enough. That's going to get worse PARTIALLY because the climate industrial complex compels the OEMs to use smaller and smaller turbocharged engines - some of which require PUL and partially because of the backlash against automotive electrification which in part is creating the current enthusiasm for higher performance 'gasser' cars and trucks.