I have a 2000 Avalon with over 60,000 miles on it, and have never had anything in it but regular gas. At about 65 mph on the highway it gets just over 30 mpg, and has great performance. I have also been looking at the new Avalon with the modified Tacoma engine in it. Unfortunately, I don't like the increased compression ratio, which could mean pre-ignition and knock with regular gas when heavy demands are placed on the engine.
For the younger viewers out there, although most of you seem to be very knowledgeable, a little background about gasoline might be interesting.
About 60 years ago when we were building "hotrods", almost every car ran on regular gas. We used to have the heads on the engine machined to increase the compression on the engine, thereby increasing the volumetric efficiency and output horsepower of the engine. Unfortunately, at ratios of 10-to-1 or greater. the gas would pre-ignite and the engine would knock or ping when accelerating heavily or speeding up a hill. To stop the pre-ignition of the highly volatile regular gas we added a "lead tetraethyl" compound to the gas. This diluted the gasoline, slowed ignition, and ran better in the engine.
We had "regular" and "ethyl" gas. They cost about the same price, since there was actually less gasoline in the ethyl. Then some brilliant marketing people discovered that if they renamed the gas "premium" or something, they could charge more money for it. There is actually less energy in premium gas than in regular gas.
However, getting back to the initial issue, the modern computers with knock detectors will modify engine timing and other parameters to eliminate the knock - at the cost of reduced horsepower and torque. But like the gentleman said - you are hoping that the computer is working.
I don't know if the ping or knocking will harm the modern engine, but it is probably not good for it, since the modern aluminum engines are designed to closer tolerances and are not as robust as the engine in my old '29 model A was. We would simply let-up on the accelerator when we heard a knock or ping and never had any trouble with the old engines, except maybe a broken valve rod now and then, but those were flat-head engines.
I'll probably buy a new Avalon, and will probably run regular in it. Good Luck.
Graybeard