A straight weight oil and a multigrade oil are very different beasts.
But, to be specific, your 30 wt example must meet 30 grade at every temperature from room temperature and above. The xxW-40 must meet 40 grade at every temperature from room temperature and above. The 30 grade oil does not have a winter weight rating (there is such a thing as 30w-30, but your example doesn't qualify), so the 10w-xx component cannot be compared directly.