O2 sensors are usually good to 100k miles (160kkm) at which point they start getting tired. If you've not replaced them, it's something I'd consider doing, especially on the before cat side, as these will cost you fuel mileage in the long run. Don't wait for them to throw a code, replace them at the proper mileage interval as a "general maintenance item&...
It could be that your fuel gauge is a bit sticky in its movement and doesn’t respond to very small changes. When you turn the ignition off and it drops to zero and then turn it back on, the large movement sets it in the correct place.
Try tapping the gauge just before turning the ignition off and see if the needle moves.
Your engine does not take much fuel to start. If your fuel level gage shows less fuel from when you shut it off to when you start it means you have a faulty sending unit that has a float to tell you how much gas is in the tank, or you have a fuel leak somewhere.