Hi,
Oh geez! This topic always seems to generate *a lot* of opinions!
The benefits of DOT5 are pretty widely known and I won't go into it much farther than to say it's much kinder to paintwork and metal than other types of brake fluid. It's less corrosive and also less prone to moisture absorbtion.
Personally, I don't use DOT5 because I've found it tends to give a softer pedal in some circumstances and it can be a lot harder to get properly bled. This is because it's more prone to capturing air when being poured in. (To help get around this, stand a screwdriver's blade in the MC reservoir and trickle the fluid down it when filling. It will get fewer bubbles. Even then, it might be best to let DOT5 sit overnight so that any air bubbles have a chance to gather together, and then re-bleed the brakes.)
It's also said that DOT5 is generally more susceptible to high temps, although I think today's formulations are better at this than in years past. I can't really say I've ever driven a car with DOT5 hard enough to take the fluid to its limit.
It's been pretty well documented that some MGB brake servos (Girling if I remember correctly) fail frequently when DOT5 is used. This might be related to water/moisture "globulizing" in the fluid, a characteristic of DOT5.
Some brake parts manufacturers void the warranty on their parts if used with DOT5. This seems to be changing, but is worth checking into when you buy parts.
All the seals in a brake system that's been used with DOT3/4 should be changed out before using DOT5, or there is a danger of seal swelling problems.
When converting from DOT3/4 to DOT5, it's important to not mix the two. So, a system needs to be flushed out well, as already noted.
My personal favorite brake fluid is ATE Super Blue Racing. Why? Well it's DOT4 and has a high temp rating (536F dry, compared to 446F dry for Castrol GT LMA), but so are many other types of fluids.
In truth, the color of the stuff is the main reason I like it!
As implied by the name, it's a bluish color that's highly visible in semi-transparent brake fluid reservoirs, as compared to more common, transparent fluids. A visual check of the fluid level is quick, sure and easy with the ATE Super Blue. Plus, it's possible that a stray system leak might be easier to spot and/or distinguish from other fluids, too, although I haven't really had this happen yet.
Sure, there are other DOT3/4 brake fluids with this coloration or similar. And, there are some with even higher temp ratings. But some of those tend to be hellaciously expensive. (Castrol SRF: $70 liter and ARP 660PF: $29 half liter!) ATE is more expensive than some, but not bad... it was about $12-15 a liter, last time I bought it.
You might point out, the convenient coloration is not really much of a consideration in earlier TRs and many other old cars that have solid metal brake reservoirs where the fluid can't be seen without removing the cap. I just can't justify having a selection of brake fluids in the garage cabinet. It's easier, for me, to have one single type of fluid on hand for use in modern cars, my motorcycle and the old TR.
By the way, ATE is also available in an amber coloration. Their "Type 200" is otherwise the same, to help insure a complete job when flushing the system with new fluid.
A final consideration, compatible DOT 3/4 fluids are very widely available... nearly everywhere. So, if a bit of it is ever needed in an emergency situation many miles from home, it's far more likely a local 7-Eleven (or whatever) will have DOT3/4 than DOT5. (Keep in mind that DOT3/4 should never be mixed with DOT5.)
So, that's my rationalization for what I use. Others will have their own reasons for their choice, and whatever that might be will do the job for them, I'm sure. I can't go so far as to say there is any such thing as a "correct" brake fluid. In a car that's gets out of the garage only occasionally and is not driven particularly hard, DOT5 might be the most "correct". For my cars and motorcycle, it's not.
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p.s. Note that ATE Super Blue Racing is *not* U.S. DOT approved... because of the color of all things! Seems that some DOT5 also use purple or blue to distinguish them from clear/amber DOT3/4 fluids. Otherwise, ATE exceeds DOT3, 4 standards easily and is compatible with those fluids, but certainly not any DOT5, of any color.
Because it's not DOT approved, it might void a parts warranty. Here's a website that compares some types of brake fluids, with extensive cost comparisons that really reinforce my decision to use ATE!
https://www.swedishbricks.net/700900FAQ/Brake%20Fluid%20Comparison.htm
This site also mentions DOT5.1 fluid, which most certainly is *not* compatible with DOT5. DOT5.1 is a higher grade than DOT3 or DOT4 and is compatible with those fluids. IMHO, the labelling of it is a recipe for disaster and it would be better called "DOT4.1" or something like that. Oh well, they didn't ask me about it when they were coming up with a name.
Hi Guest!
Hey - did you know if you click on the title of a thread it will take you to the first unread post since you last visited that thread?
but were afraid to ask: 
STOP!! Never post your email address in open forums. Bots can "harvest" your email! If you must share your email use a Private Message or use the 
smilie in place of the real @
Pretty Please - add it to our Events forum(s) and add to the calendar! >> 
