i think with multigrades, it's not the thickness, but the lubricating properties that give it its designation.  for example, at 0*, a 10w30 oil will lubricate like a 10 but lube like a 30 at operating temp.  for the most part the thickness seems to be in between.

and another interesting thing to think about:  the wider the viscosity gap, the more polymer viscosity modifiers are in the oil and less actual oil.  because of this, *in theory* at operating temp, straight 30 would be better than 10w30, which would be better than 5w30.  however you have to account for dry cold starts which the 10w30 and 5w30 would give much better initial lubrication.  it's all a bunch of trade-offs, that's why there's so many different oil choices