I rotate between 89 octane and 91..

http://blog.consumerguide.com/do-i-really-need-to-use-premium-gas/

Herbert said that both vehicles (an RC 350 sports coupe and an NX 200t compact SUV) will run on regular-grade fuel without damage. But since both the advertised power ratings and the EPA fuel-economy figures were attained using premium, that’s what they have to put as the required fuel. However, he also warned that continued use of regular-grade gas would cause the engine’s computer to back off on the ignition timing—thus adversely affecting both power and fuel economy—and that it would continue to do so for a time even after the car is filled with premium.

It’s important to note that Herbert’s advice only applies to Lexus vehicles, as he obviously can’t speak for other manufacturers—and we can’t, either. And while our stance is that you should use premium-grade fuel if it’s stated to be “Required” or “Only,” here are some thoughts if you accidentally fill it with regular.