DOT ratings for brake fluid are strictly temperature rating...which is why there is such a problem with actual TYPE of fluid and all the compatibility issues!
As Glider says, ethylene glycol based brake fluids have been standard fare for years...only problem was it absorbed water over time which lowers boiling point and water settling to the lowest parts of the braking system causes rust/corrosion. Silicone based fluids were introduced in racing circles for their temperature advantage (hence DOT 5) but was not compatible with ANY of the ethylene glycol versions, and though "inert"...silicone is a contaminant that does not degrade or clean up well in the environment (like teflon)...some military specs even require silicone NOT be used in their equipment).
Since "Synthetic" is all the rage now due to it's purity (being chemically all man made with no impurities) it is a great advertising/marketing ploy...but compatibility to existing fluids I doubt has been researched fully over YEARS of testing...so I would stick with conventional as the "--jury is still out". Just remember the problems with MTBE, alcohol and other additives added to your fuel in the interest of stretching "YOUR" fuel (it actually expanded the oil companies' wallets)...and 10 years later we have pollution that leaches into ground water, turns your fuel lines and fiberglass tanks to mush...JMO but I would just wait and see. :no