Sort for DOT3. Look for the highest wet boiling point. It seems that the Lucas product is the best on this list.
Don't be fooled by "synthetic" on the label. Brake fluid is polyglycol ether based. It's all synthetic. DOT4 has some borate compound in it to raise the wet boiling point, but that also shortens the effective life; it absorbs moisture more easily. DOT4+ is an unofficial spec for higher rated fluid, as is DOT5.1. DOT5 is completely different, silicone based, must only be used in a system made for it, and never mixed with a different fluid.
All brake fluid absorbs moisture from the atmosphere.* When the moisture content is too high the high heat from hard or emergency braking will cause the moisture-laden fluid to flash to vapor. Your brake pedal goes to the floor and you have no brakes. It happened to a friend towing his horse trailer down Mt. Adams. He got his truck stopped using low gears and the parking brake, took a lot of deep breaths, looked around and saw nothing wrong, got in and tried it again. He had a brake pedal again. He crept home and went to his mechanic. Confirmed, moisture in the fluid. He now changes the brake fluid every couple of years and shifts to a low gear for long downhill grades.
*Polyglycol ether fluids allow the moisture to mix in the fluid so it all is emptied when the fluid is flushed. Silicone fluid doesn't mix with moisture, the water puddles in the low spots.
