There is no difference between an oil tanker that is full or empty. Or if there is, the difference is imperceptible
I just tried this. For me empty Human Oil Tankers looks full when their sprites are horizontally/vertically. And empty when they’re diagonally.
In all cases it is extremely difficult to tell the difference too
it is not present on my end. my oil tankers is always using the right graphics.
I also saw this critic in a video on YouTube recently, so you are not alone.