Using regular unleaded gasoline in a Lexus RX330

Lexus recommends a 91 Research Octane Number (RON).

Here is additional information related to octane ratings and standards worldwide.

Gasoline has chemical components that have anti-knock components. Anti-knock components elude to chemicals within the gasoline that prevent the fuel from pre-igniting under compression before the spark fires. If your vehicle starts to pre-ignite it will sound like there are marbles bouncing around in your engine or a loud rattling. The sound is generated by combustion occurring too early in the combustion process as the piston is rising on it's compression stroke. Due to compression and lower octane the fuel charge initiates combustion early and the expanding gas is working against the piston upstroke. The sound of rattling is the piston slapping in the cylinder.

If this begins to occur you would want to switch up to a higher octane rating. Increased temperatures with Summer could increase the chances of knocking with the lower octane rating.

Agreeing with @DucatiKiller here for the most part. There are a couple of things of note though ...

The reason your engine seems to be running fine is because the computer will detect the "pinging" before you ever will through the use of a knock sensor. When the ECU detects pinging, it will start pulling timing (retarding your timing) until it stops pinging or it gets to a preset limit. This allows the engine to run without any damage to the engine. What happens when this occurs, though, is by pulling the timing, it is also detuning your car. It is making it so it isn't running as efficiently or as strongly as it was designed to do.

You will notice a drop in performance and a drop in fuel economy by using the lesser octane. As long as the ECU can keep pulling timing when encountering the pinging, you'll not have any detrimental effects to the car mechanically. Once it reaches the pinging level where it's not allowed to pull anymore timing, you'll start seeing the mechanical effects, which could be anything from something minor to complete engine destruction (and I'm not be melodramatic when I say that). You can easily push past this point by rapid acceleration (stomping on the gas pedal or going to Wide Open Throttle) or even during climbing a steep incline.

What it comes down to is this: Use the lower octane fuel at your own risk. Know you are risking the long term health of your vehicle by using it. Also realize you will not have the performance or fuel economy you are used to with the Super.

According to https://lexus2.custhelp.com/app/answers/detail/a_id/8129/~/which-fuel-grade-should-i-use-for-my-lexus-vehicle%3F

2004-2005 RX 330 has a "Recommended Minimum Octane Rating" of 87 2006 RX 330 has a "Recommended Minimum Octane Rating" of 91

I am looking for the answer too and this is what I found today, having lost the original user manual.

It will run fine , the knock sensor and computer will retard spark as necessary. You will lose peak performance. I have a 2011 Murano 3.5 L , I used regular in it for 80,000 + miles, never a problem. With the pandemic situation , I am driving less and using little fuel, so I went to premium. I have been impressed by the power improvement. No difference going down the freeway at 75, but it gets to 75 very fast.