Why to Use a fuel with the lowest required octane in the car?

Octane ratings have nothing to do with fuel economy or power. The octane rating only indicates how much the fuel air mixture can be compressed before it ignites without the spark plug firing in a hot running engine. Some really high compression engines require higher octane fuels so they don't knock when running. If you don't have a sticker under the filler lid that says you have to use one of higher octane fuels, then don't use high-test.
It used to be that using a low octane fuel in an engine designed to use high octane fuels could damage the engine.
Nowadays, engines are all computer controlled and most high octane engines will re-tune themselves on the fly so that they don't knock if the wrong grade of fuel is used.

It may not be bad gas, but it could be the wrong octane rating for that engine. Check your owner's manual and see what octane is recommended for that engine. If you're running 87 octane and it requires 91 or higher octane, then you need to be running the higher octane fuel. By running lower octane fuel in an engine that requires higher octane, your getting pre-ignition, the knocking sound you hear. What's happening is that as the piston moves up on the compression stroke, the gas/air mixture is compressing and as the pressure goes up, so does the temperature of that mixture. It's possible that temperature can get so hot that the gas/air mixture ignites from the high temperature and not from the spark plug. Higher octane fuel will help this since higher octane fuel is harder to ignite. I say try running some 92 or higher octane fuel in your car and see if you still hear the knocking.

No, it will do no harm at all. As with the majority of modern engines, the car's engine management system automatically adjusts engine settings (the ignition timing curve via the anti-knock sensors) to suit the lower octane ("regular") fuel. The only penalty is that the engine will not have quite as much power as it could otherwise have.

The rules are simple: on vehicles which state only use "super" that's all you use and such cars are now unusual. On vehicles which advise that "super" is recommended, you make the choice; no damage will occur but the engine will not reach peak performance on anything but "super"; this may also mean that fuel economy is not as good but you then need to weigh up the possible loss of economy against the higher cost of "super". On vehicles which recommend you use "regular" using "super" does no harm but offers no benefits at all.

"Super" is not a measure of how much energy the petrol has, it's just an indicator of the octane level. Higher octane fuel is more resistant to premature detonation ("knocking") and allows the engine timing to be set in a way which is more favourable to economy and high speed running. If an engine's management system lacks the ability to exploit the higher knock resistance, then there's absolutely no way for it to get any benefit and it's just a waste of money.

In practice, and especially in the USA where speed limits are often lower than elsewhere in the world, it's unlikely that you'll notice the difference under most conditions when using regular in your Lexus.

I hope that my reply has been helpful, please take a moment to rate my answer.

HI. i recommend using the premium in this vehicle. This will ensure optimal performance. The fuel system will accept the regular blend as well but, this is not what this vehicle was engineered for. the regular blends octane level is not sufficient for optimal acceleration and overall performance.

Please rate and god bless:)

Best way to check is by looking at the gas cap. If it does not say "Premium fuel only" you should be safe. Higher octane rated fuel is used for higher compression motors, and I dont belive that your engine (my guess is the 5.4 V8 should be around 9:1 compression) needs the high octane.