Here's a very basic explanation of multi weight oil from edmunds.com:
"...20-weight oil would remain 20-weight oil at all temperatures, where as 10-30 would be 10-weight oil when cold and 30-weight oil once warmed to normal engine operating temperatures..."
        http://www.edmunds.com/ownership/driving/articles/44791/article.html

I found this somewhat interesting:
        http://info.gdb.org/~djw/411/oil.html
 
I personally use straight weight oils in my aircooled VW because that's what the manual calls for, www.thesamba.com has some of the owners manuals online.  I have heard but cannot substantiate claims that multi weight oils foam in aircooled motors due to higher operating temps. I would *guess* that *if* this occurs it would be a byproduct the polymers breaking down, polymers are not used in single weight oils.

Dave
251.org
'87 Vanagon GL Wolfsburg (186k)
'61 Microbus (80k)
'88 GTI 2.0 16v (162k)

>Still no answers on why anyone should use straight weight over multigrade?
>
>At higher temps, is 20-50W the same viscosity as just 50W?