Had a few Lexus vehicles over the past few years and I will say that I go between on all of them. I have no problem putting Regular octane in the car. I have not noticed an impact on my overall mpg. Nor do I really notice a difference in performance. I will say that I am not ever heavy on the gas so any advantages with higher octane may not have been noticeable. I put regular in our 2007 GS430 and I have even thrown in a couple regular tanks in our 2013 GS350.

I am sure the higher octane helps Lexus achieve their posted HP and torque numbers, but in real world everyday driving I would be hard pressed to say if I notice a difference.

Thanks for everyone's input. It sounds like we can get optimal performance by using premium grade fuel, but the car will run fine on the cheap stuff too. I plan to continue to use premium fuel.

Because i have asked this question, some astute folks have figured me out. Busted, I am living beyond my means and will probably have to sell a kidney. It's worth it though to have a fun car...

Thanks, Arkie

I wasn't directing that to you specifically, more-so in general. If anything I would say it to the cheap salesmen that get these cars and do it themselves(or so they say..) and then tell the customer you can do it. Just the difference in price doesn't justify the potential effect it could have on your car.

Arkie,
That is so sad, and at Christmas time too. Where can we send a donation?

Premium FTW...

I got this from my TIS bible:

You must only use unleaded gasoline in your vehicle.
Select premium unleaded gasoline with an octane rating of 91
(Research Octane Number 96) or higher required for optimum engine
performance. If 91 octane cannot be obtained, you may use unleaded
gasoline with an octane rating as low as 87 (Research Octane Number
91). Use of unleaded gasoline with an octane rating lower than 91 may
result in engine knocking and significantly reduced performance. Persistent
knocking can lead to engine damage and should be corrected
by refueling with higher octane unleaded gasoline.
At minimum, the gasoline you use should meet the specifications of
ASTM D4814 in the U.S.A. and CGSB3.5-M93 in Canada.

This is true as I had a customer with an IS350 who came in with a check engine light . We diagnosed it, and found a code for misfiring and a code for engine knocking. We told the customer that they would need to use 91 octane gas or higher for their condition to improve, and hopefully he has, because I haven't heard back from him for a while now.

On the Bimmerfest Board the same debate came up. My response was like yours. But in CA there is only a 20 cent difference between the top tier gas and the bottom;

Let's say, again, you drive 15,000 miles a year and average 23 MPG. In a year you will use 652 gallons of gas. If you pay 20 cents more for premium gas you will spend $130 more in a year or $10.83 a month. I'll spend that money.

Lol I've seen this from 2GS to 3GS to 4GS. If premium is recommended why put in anything else?

Sure you can use regular. You can also eat Waffle House everyday and see how that works out

I find this hard to believe??? With the technology built into the computers running our engines that sense lower octane gas and adjust accordingly to avoid engine knock.

For the Record, I use Premium!

What is the TIS bible?

Typically, the engine's knock sensor will detect pre-ignition and retard the timing. That still requires that knock (pre-ignition) occur in the first place. The result of the knock senor commanding the ignition system to retard the timing is decreased power output and increased fuel consumption.

The GS 350 uses a high compression (11.8:1) engine. The higher the compression ratio an engine has, the hotter the engine's combustion chambers get. Higher combustion chamber temperatures mean the air/fuel mixture is more likely to spontaneously ignite (pre-ignition). The spark plug should initiate the ignition of the air/fuel ratio, not remnant heat within the combustion chamber. Higher octane fuel has a higher resistance to pre-ignition. Since high compression engines generate higher heat levels, using a fuel which is less likely to ignite at higher heat levels (a higher octante rating) is appropriate.

In summary, use the octane specified by the manufacturer.

It's an official Toyota/Lexus/Scion online database of information, parts, labor, diagrams, etc.
http://tis.toyota.com/
You have to work for a dealer or subscribe to it for access.

I understand, but that is from the horse's mouth(Lexus).

Premium only!