Does the car know the difference? I would pay the difference to make the difference. I'm just glad it doesn't cost more here, of course. But we have seen brief $4 / gallon here in Utah a few times. In Cali it rarely goes below $4 / gallon anymore, for top tier premium. Sounds cheap when compared to the EU, of course. But if the engine "cares" over the long term, it doesn't make sense to put thousands out for a car and then slowly kill the engine with lower priced gas. If you have any doubts about hurting the engine with lower octane than premium, it also doesn't make sense to go against what you feel is happening when you conserve several dollars each time you fill up.