In short: yes, at least according to automotive expert Jeffrey Faircloth, who spoke to Motor Trend about the topic. The more long-winded version suggests that manufacturers recommend premium fuel for a reason; namely that an engine is specifically designed to function best when using a certain grade fuel. So while using a lesser grade may not necessarily cause outright harm, fuel efficiency and performance may be compromised. One may also look at the fuel-grade discrepancies in different parts of the country. Whereas 93 octane is the common premium fuel in the Northeast U.S., West coast drivers operate the very same vehicles on 91 (often the highest octane available), with no particular ill effects.