Yeah, just make it work like a car's fuel tank. It has a gauge to say how much is in it. It has a hole so you can add more. Some cars will guess how far you can drive, give or take, based on how much fuel is in the tank. If the fuel gets very low, a more obvious warning will pop up in case you weren't watching your gauge. But otherwise it just keeps driving in the meantime and if your car needs high octane and you give it low, it will try to run it anyways and if it fucks up the engine, then that's on the user.