Can o' worms there.

I was taught tha units are never to be pluralised, even though it happens nearly all the time, "Five miles" etc. I don't recall the reasoning, but I suspect that it has something to do with the fact that the unit is abstract and not tangible.

eg When descibing the maximum amount of water that a "Five Gallon Bucket" would hold, you'd say "Five Gallon", "40 Pint" or "22.73 litre". Each gallon, pint or litre is abstract.

Now if you actually filled the bucket and wanted to describe how much water you had...