ml or oz?

When making up a bottle, do you go my millilitres or ounces? The instructions on the box make such a big thing about using the right amount of powder - making sure it is a level scoop - but I've just noticed on the bottle that there's quite a difference between 6oz and 180ml, which are both shown as 6 scoops.



Sorry, bit of a pointless post, but just wondered what measurements others were using and whether it made a difference?