20W-20 is is a straight grade that was pretty popular until the '80s. Straight grades were cheaper, so even though there were more advanced products they did their job and were a good value. The reason it looks like a multi-grade is because the SAE has both a maximum thickness when cold winter rating and minimum thinness when hot summer rating for grade 20. So an 20 oil that met both criteria could be labeled 20W-20 to show it.
If you can post up clear pictures of labels I can probably help you with a lot of your questions.