I agree that it is extreme to treat any extra demand for dollars as a result of oil contracts being settled in dollars as being as much as one day's transactions, but it is both a conservative value for the purposes of the argument - the amount of extra demand is certainly less than that - and a convenient one, since the information on the value of oil sold per day is readily available.

And by contrast, should China change the weight in its Singapore peg market basket, it would easily amount to a more substantial impact on FX markets than the dollar value of one day's worth of oil.

I've been accused of being a Marxist, yet while Harpo's my favourite, it's Groucho I'm always quoting. Odd, that.