SHORT ANSWER
When using a traditional glycol based fluid, use a DOT4 specification product such as Castrol LMA or Girling Universal. If the brake system has been converted for use with silicone fluids, any DOT5 (Not DOT5.1) rated product is acceptable.
LONG ANSWER
Traditional brake fluids such as Castrol LMA and Girling Universal use a glycol base that renders them highly hygroscopic, meaning that they will absorb atmospheric moisture, which can cause corrosion in the wheel cylinder bores and pistons, leaks in the hoses and seals, not to mention reducing braking efficiency and threatening the safety of the entire braking system. Even quality products can absorb one percent or more moisture per year of service life, increasing as the fluid ages to levels that can seriously affect the safety and performance of the brakes. When using traditional fluid, always specify a DOT 4 fluid, since this specification will absorb less moisture than fluids manufactured to a DOT 3 standard, remembering to completely flush the system and change the fluid at least every other year, and more often in areas with high ambient moisture.
One solution to avoid these concerns is to use silicone brake fluid rather than traditional glycol-based fluid in the braking system, since it absorbs very little water and will not damage paint work if spilled on the car. A newly rebuilt and converted system should outlast one using glycol based fluid by several times, although it is important to bleed the fluid on a regular basis.
Despite these advantages there are some factors to be considered before converting your braking system. Because these modern fluids contain dissolved air (different than trapped air) it may result in a softer brake pedal, particularly near the higher end of their temperature range, which explains why they are not used in racing applications where pedal modulation is an important issue.
Silicone fluids also possess a higher viscosity than traditional fluids, which combined with a greater tendency to absorb air, leads to slow fill rates and retention of free air entrapped during filling process, which makes bleeding the system difficult. Because air bubbles do not easily dissipate in silicone fluid, special care must be used to prevent them from forming when it is poured and the system is bled.
A conversion to silicone fluid should only be done as one part of a complete brake system overhaul, complete with new or freshly rebuilt brake calipers, wheel cylinders and master cylinder. Silicone fluid should never be added to a system that contains even minute amounts of glycol fluid or other contaminants. Merely bleeding the system is insufficient, as there will be pockets of old fluid and sludge that will not bleed out.
Silicone fluid also tends to concentrate any residual glycol fluid, moisture and sludge into slugs, instead of allowing their dispersal throughout the system, as is the case with traditional fluids. This can lead to relatively severe but localized problems, rather than the more general system deterioration experienced with glycol fluids. This may be a factor in reports of leakage when silicone fluid is used in non-rebuilt systems that had been operated with glycol fluid, since a “new” system filled with silicone fluid will require very little maintenance for years, other than for standard fluid changes.