up to about 2 - 3 litres i wouldn't be to worried if its always done that, my s2k used to use about 3 litres between services, but then thats what they do on the earleir engines.
who ever was getting through 9 litres i would be concerned, but i would want to know what they were putting in the engine. before i made comment.
on the zed i would say anthing up to about 2 litres, maybe 3 i wouldn't be to concerned.
i think what you have to worry about is if there is a dramatic change from the norm. then yes worry, but if the consumption is the same on every service, then the engine hasn't changed its operating function.