Here are a couple product safety sheets from oil companies, and they say to avoid leaving it sit in the sun, but they don't say why, which leads me to believe it could be just because they want to avoid heat buildup or container bursting.
So far I haven't found anything saying that it actually degrades from sunlight.
http://www.pennzoil.com.au/downloads...d_Dot_4_AU.pdf
http://www.federalmogul.com/NR/rdonl...P_DOT_4_UK.pdf