Holy fuck are you kidding me?
When it is 32* it IS a 5 weight. That better? When it is 210*, it IS a 20 weight. A 20 weight at 32* will be too thick to get to any small clearances and what not to lube parts. So essentially what it means is that at 32* the 5 is thinner than a 20 weight also at 32*. At 210*, the 20 weight is thicker than the 5 weight. Get it now.