DOT 3 and 4 are glycol based, chemically similar to antifreeze. The only difference is the wet boiling point, 4 is higher than 3. DOT 3 is the same as the Delco Super 11 that was the oem fill. DOT 4 is also ok.
DOT 5 is a silicone based fluid, used mainly in motorcycles. It has a higher boiling point that the glycol based 3 and 4. It also does not absorb moisture like the glycol fluids. There are potential compatability concerns when using in inautomotive brake systems that did not use it as oem. As long as all of the rubber components are certified for use with silicone fluid, it should be ok.
There are a couple of warnings if using the DOT 5, one concerning the mentioned compatability. If the rubber is not DOT 5 compatible, it can cause premature seal failures. Some cases, the seals do not swell enoung, others it swells them too much and makes them go mushy.
The other issue, which is talked about in Bob Adlers column in the current issue of Vintage Truck, is the non-hygroscopic nature of the silicone fluid. If moisture does get in the system, instead of blending in the fluid, it makes a "slug" of water, usually at the low point in the line. This can cause a mushy pedal, especially whan the brakes get hot. Another problem, which Bob talks about, is the potential for the water to freeze in the lines, making the brakes non-functional.
There is one other fluid on the market, DOT 5.1, which is a glycol based fluid with a very high boiling point.
My personal opinion, based on both my experiences with using DOT 5 and my job experience as a brake mechanic, stick with glycol.
