DOT 5 is a silicone-based brake fluid (contains at least 70% by weight of a diorgano polysiloxane). Unlike polyethylene glycol based fluids, it is hydrophobic. DOT 5.1, like DOT 3 and DOT 4, is a polyethylene glycol-based fluid (contrasted with DOT 5 which is silicone-based). Polyethylene glycol fluids are hygroscopic and will absorb water from the atmosphere, which is necessary to prevent sheer and undiluted water in the braking system, which is very corrosive. Also, water droplets can freeze in the pipes, thus blocking the system.
Glycol based fluid absorbs moisture, and disperses the moisture throughout the system and contains corrosion inhibitors. Silicone fluid does not allow moisture to enter the system, but does not disperse any that is already there, either. A system filled from dry with silicone fluid does not require the fluid to be changed at intervals, only when the system has been disturbed for a component repair or renewal. The United States armed forces have standardised on silicone brake fluid since the 1990s. Silicone fluid is used extensively in cold climate, particularly in Russia and Finland.
With DOT 3, DOT 4 and DOT 5.1, moisture diffuses into the fluid through brake hoses and rubber seals and, eventually, the fluid will have to be replaced when the water content becomes too high. Because Silicone brake fluid does not absorb and disperse water throughout a brake system, if water does enter the system, (as in the system being disturbed for a component repair) the system can have pockets, or sections of corrosive moisture. Once moisture enters a silicone based brake system, corrosion is inevitable, although if the system is "not" disturbed, the system can go many years without service.
I'll stick with DOT 4
Bogie is the man, and I should learn not to mess with him