Not sure what km=in miles, but I know you can go over 5k miles on almost any oil, whether synthetic or not. And I know that's more than 5k km. It just depends on how good your engine is and what conditions it sees. People have done real tests and you can go a suprisingly long amount of time, especially on synthetic with a good engine. My mobil1 doesn't change color much at all after 3k miles and I have no qualms about going 5k miles or more. I just change it when I feel the urge and it's always pretty clean it when I change it.
