They probably differ mostly in additive package and use the same base oil. It is also a multigrade and some people feel that the 0w30 (not multigrade) quiets the engines down more.

Here's a thread on Bob is the Oil Guy about just what you're asking:
http://theoildrop.server101.com/cgi/...c;f=1;t=015349

What the API symbols mean:
http://api-ec.api.org/about/index.cf...02003002000000
http://api-ep.api.org/filelibrary/AP...Guide_2004.pdf

There is some good stuff here, although the site is fugly:
http://www.bobistheoilguy.com/

I'm actually gonna try Chevron Delo 15w40 next change after reading a bunch of stuff and this thread about Shell Rotella 15w40 fixing the Isuzu 3.5L oil consumption problem for somebody:
http://forum.planetisuzoo.com/viewtopic.php?t=7033

I'm running Amsoil XL 5w30 right now and don't really care for it. It's hard to tell at this point, though, since it's only the first oil change I've put in it and it is loaded with UV dye.