DOT3 (&4) are glycol-based, while DOT5 is silicone-based.
In theory, DOT5 is superior to anything else as it has a higher boiling point and doesn't attract moisture, but for the life of me I have never had luck with the stuff. It seems to hold air which leads to spongy brakes and your system has to be totally rid of the old fluid as the two aren't compatible.
Valvoline Synpower makes a DOT4 that I can recommend. I wasn't satisfied with the brake performance in my '51, so I flushed out the DOT5 and replaced it with the Synpower DOT4 and all my problems went away.
