Owner of a 2019 NX300 and was browsing forums wondering why the definitive final answer to this was.
It is clear that this thread will pop up every 2 years or so and a 10 page debate occurs which leads to no conclusion. Usually its just people saying to either follow the manual (since it says required) or people saying that its fine to put regular gas in for just town driving since the newer vehicles have engine knock sensors that detect potential problems. If anyone could in a structured manner describe the potential consequences of regular gas in the 2019 NX300 that would be great. What about midgrade?

I on the other hand have been driving with regular gas and have found no problems. Although this is mostly because i did not realize premium gas was required until recently. I am also not thrilled that premium gas is almost 1 dollar more than regular.
In my experince driving many miles with regular gas I have found

• No engine knocking
• No signs of poor performance
• No engine lights or other computer warnings

Anyone with information regarding their experinces, please add.

 

I have found the exact results you have that is why I only use regular.

We want to treat our vehicles the best and if putting premium gas does that for you by all means go for it. Have used 87 octane in our 2015 RX450h and currently 2017 which both premium is recommended. Experimenting with the other two levels, I have noticed zero difference in gas mileage or performance. I have used 87 in cars that have required premium for over a million miles and have not had one problem.
I always revert back to two MIT graduates that are car mechanics who had a syndicated radio show called Car Talk for 35 years.
http://www.cartalk.com/content/premi...egular-0#myth4

From the USA Today:
The only modern engines that should really need premium are those with superchargers, which force-feed fuel into the cylinders. "You're driving along and just tramp the gas and the knock sensor cannot sense the knock fast enough in some cases," because the supercharger boosts pressure so fast, says Bob Furey, chemist and fuels specialist at General Motors.

Burning regular when the owner's manual specifies premium won't void the warranty, nor damage the engine, even the most finicky automakers say. "You're giving up perhaps just a little bit of performance that a customer wouldn't really even notice, it's so slight," says Furey.

 

Your engine is design for high octane, so it will keep trying to advance timing and when a slight knock is detect, undetectable by the driver, it pulls back timing due to the low octane gas. That slight knocking can cause a incomplete combustion cycle which over time can cause carbon build up.

 

The Lexus engineers recommend premium fuel for a reason.