It sounds like the fuel level sensor is failing / has failed. The ! light on the dash is not from doing the oil change too early. It's from not resetting the service light when the oil change was performed. Please let us know about the fuel gauge situation.

 

This has been randomly happening to me for the last 6 or so months.

Year: 2014
Model: IS350 F-Sport
Mileage: 42,500 or so (owned since new, bought a month or so after US release)

My roommate was borrowing the car the first time it happened and is occasionally not the brightest, so when she came home telling me she was sure she pumped gas, but the tank was empty. After going into the store and making herself look like an idiot, I took the car back to the gas station. It had now been turned on and off twice and driven several miles, yet still said "Low Range" and reported an empty tank with the orange line instead of white. Even though you can feel the vibration of the gas flowing into the tank as you fill up, I doubted that she pumped anything. To my surprise the pump flowed for a second and stopped. I tried again causing gas to overflow out of the fill tube. Now knowing it was definitely full, I got back in the car to the report of an empty tank by the gauge. Knowing full well the tank couldn't be any more full, I left confident that there was a problem with the guage. About 5 minutes into the drive the gauge suddenly went to completely full showing a range of 290 miles (normal for my every day city driving).

I always forget to tell the dealer when it is in for service because it has always been working fine on or near my maintenance windows and I never remember to say anything even though the car has been back for two routine services since. Most of the time the gauge works fine. It occasionally shows empty after a fill up for the first 5 minutes of driving and also only appears to happen if I get gas where the tank is mostly empty (14+ gallons pumped). Having swapped full pumps in my project mustang I am familiar with the way a float switch works (if that is the way Lexus does it) and would definitely say that the float is getting stuck and after some driving is being shaken loose and allowed to float back to the surface of the fuel delivering an accurate reading.

I'm actually going to set a reminder to mention it this time since I am steadily creeping up to the end of my warranty.