Quick note: for brake fluid, synthetic isn't synonymous with silicone.
Castrol SRF synthetic brake fluid does absorb water and is classified as DOT 4. You can mix it with any DOT 4 or DOT 3 fluid (not that you'd want to). It's known for its awesomely high wet boiling point.
Silicone fluid, DOT 5, is a different animal.
As John mentioned, there's been quite a lot of discussion on the subject around here. A little searching should lead to some great info.
PC.