Thanks Exdee, I probably will go back to Lexus again and talk with them, just not sure as my car has out of warranty, and the problem existed since it still was in warranty, if they will do anything :)
Is a normal car should/shouldn't need to top up oil between services? The first time I noticed this was 11 months I got this car, while trying to push my car to 120mph (yes, but just for 1~2 minutes, and I do NOT suggest others do so, and TBH as a new driver and it's my first car, I was keened to see how fast my car goes), the low engine oil level warning was given. I immediately slowed down, and brought to Lexus to investigate and they topped up oil. I then realised that I have to top up half or whole bottle of oil between services afterwards.
I didn't think it burns oil as I never see grey smoke out from the exhaust, just some white fog during cold day, I assume it is steam (but where the water came from?) and harmless, am I right?