dot 3, 4, and 5.1 are all compatible, and are in order from lowest to highest WET boiling point. 3 and 4 are common, and if you have the choice go dot 4. dot 5.1 is more of a specialty 'high performance race' type fluid and isn't commonly available at regular parts stores. Though this is the best fluid, its a good bit more $$$ and not needed at all.
DO NOT USE DOT 5!!! It is a completely different chemical make up and not compatible with any of the above listed fluids.
Every car/truck should have its fluid fully changed every 2 years. SOme people have 15 y/o brake fluid in their rigs, but for optimal performance both in braking and parts longevity, a 2 year change cycle is the way to go. WHy not? it costs you $8 in fluid and 40min or so w/ a buddy....