brake fluid choice (long)
You may have opened a Pandora's Box of feedback, but I'll give you my opinion. DOT 4 is what I would recommend. It has a higher boiling point than DOT 3 and is glycol based, which means it is hydroscopic or water absorbent. I've read opinions on lists recently where people are recommending DOT 5 synthetic becasue it is not hydroscopic and therefore water absorption is not a property.
As it was explained to me, a real world car (as opposed to a trailer queen race vehicle) is driven in hot and cold weather, sunshine and rain, summer and winter. At best, most regualr drivers see brake fluid flushed about once a year. At worst, let's just say life of the vehicle.
If you use DOT 4 fluid, every time you flush the fluid, if done regularly, you flush all of the qater out of the system because it is entrained in the fluid. If you have DOT 5 synthetic, any water is collecting in the system lowpoint and will sit there and corrode the line. Also, until Valvoline came out wiht their DOT 4 synthetic, no synthetic fluids would mix with regular fluid.
The benefit of DOT 5 is the higher boiling point, however, it is usually only used in track cars that are getting all of their fluids changed daily or thereabouts. I have also heard that replacing DOT 4 with DOT 5 requires extensive flushing because they don't mix well and can form an emulsion (goo) on o-rings.
All told, the benefits to DOT 5 aren't worth the pain when quality DOT 4 (Castrol LMA DOT only $3/pt and widely used) will meet the need and keep the system clean if changed every year or two. The boiling point is maintained as long as the fluid is changed regularly and the water is kept from accumulating, usually from condensation in the system.
HTH, sorry it was so long, but I've seen this discussion and wanted to weigh in.