Just to be clear, 20W-50 designation means that when the oil is cold, it will be no thicker than 20wt oil. When it is hot, it will not thin out any more than 50wt oil.
A good way to remember this is the W stands for winter and not weight. I forget what the temp is for the W designation. I would guess that it's somewhere around 32F or 0C.
There actually is a 0 weight SAE designation for oil. Not sure what the specs are so if you want to know the details, a search of the SAE or API websites will probably tell you.
So for the Lookie Loos... Lookie it up.