Factory specifications call for DOT-3 fluid in DSMs. Many DSMers use Ford Heavy Duty brake fluid - it reportedly has a high temperature limit. Canadians will be happy to note that Canadian Tire sells this fluid as well, listed as "For Fords".
Brake fluids are rated according to DOT (Department of Transportation) ratings, which were established in 1972 to provide standardization. Note that DOT-3 and DOT-4 fluids are NOT compatible and may not be mixed in the same system. Silicone brake fluids are DOT-5. They are not recommended for use because they have strange temperature characteristics, and allow water to accumulate in the brake system.
All brake fluids are hydrophilic ('liking water') meaning they tend to absorb water. This decreases their effectiveness, and is a reason why brake performance decreases over time. Replacing the fluid with new, unopened fluid is the only remedy.