I just filled my new RX450h with regular fuel by mistake (not Premium). Will this cause harm to the engine? What can I do to correct this?

Nope, you'll be fine.

Thanks. I hope you are right.

I am. The engine will adjust. Same drivetrain in the Highlander Hybrid (that I have) and yet the HH doesn't require premium.

If you look under the fuel section in your manual, I believe it states that you can use regular if premium is not available.

Thanks again. Do you think it would be helpful if I poured an octane booster into the tank?

car will just have lower power and lower mpg while the gas is in there, maybe halfway into the tank you can top off with premium to get the octane up. Wont have long term damage just reduced performance

Thanks for your response.

Wouldn’t hurt to add octane booster

here is what is listed in your manual.

Yes I read that. Reason I am somewhat concerned. Thanks.

One time you’re good. Just don’t do it all the time.

Your RX will be fine. The computer will adjust for the lower octane.
If it makes you feel better, go stand in the corner for 30 minutes.

As mentioned earlier, the engine's computer and knock-sensors will probably retard the timing a little to compensate for the lower octane, but that should just make the engine run a little less-efficient for a while. You can minimize the lower-octane effect by re-topping the tank off with premium at, say, the halfway point or above. But I wouldn't worry about it....it's not likely to do much if any harm, particularly if you drive easy.

We presume that. You definitely do not want to run a lower octane permanently

Got it! The Lexus dealer here in Calgary just suggested the same.

Nothing to worry about. In fact, you could run regular in that all the time and be fine.