ColinL

funky, let's see some proof of what you're claiming. all polyglycol-based brake fluids are very hygroscopic and there should be no meaningful difference between two fluids that are DOT 3, 4, 5.1. silicon-based fluids that's a different story but we aren't talking about those. and RBF600 has a higher wet boiling point than ATE super blue as well so I think you're washed up.

on paper.

but in the real world, I completely agree with Adam-- there will be no practical difference observed between any of the top level fluids.

Funky

Talk to some shops that sell both, where the owners actually race and they can fill you in on all the idiosynchrasies of these fluids, they are very different. As far as I know, there is no standard test to measure varying levels of hygroscopicity of brake fluids, all information I have gathered has been anecdotal. I have used both fluids on the track, and can contribute similar anecdotal evidence.

If the hygroscopicity of all dot4 brake fluids were the same, Castrol LMA would not exist. (LMA = Low Moisture Activity, it is designed to compete with ATE Super Blue for longevity)

In the ultimate display of messageboard'ism, I'm going to quote an internet webpage and pretend it's the inarguable truth. This taken from http://www.shotimes.com/SHO3brakefluid.html

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Castrol LMA is very good at rejecting moisture and may be kept in your brake system for a couple years. The LMA stands for "Low Moisture Activity". This is the minimum quality stuff that I would use in my Impala. It comes in plastic containers which do not have a long shelf life. Don't buy lots of this stuff at a time because moisture can make its way through the plastic containers.

Ford Heavy Duty DOT 3 is VERY inexpensive and is popular among racers because of its excellent dry boiling point. It absorbs moisture quickly, but the racers don't care since they change their fluid frequently. Comes in metal cans so it may be stored. I would not use this in my Impala for the street.

ATE Super Blue Racing and ATE TYP 200 are the same brake fluid in two different colors (blue and amber, respectively). BMW recommends this brake fluid for their street cars because it, like Castrol LMA, absorbs moisture very slowly. The advantage over LMA is that ATE has a much better wet boiling point. You can put this stuff in your car and forget about it for a long time. An excellent choice for a weekend track car which also sees regular street duty. Comes in metal cans. This is what I use in all my street cars.

Motul Racing 600 is a very exotic and expensive synthetic fluid with high wet and dry boiling points. I use this exclusively in my race cars. Too expensive for the street and requires frequent changing due to its hygroscopic nature. Sold in plastic bottles. It is not suitable for the street because it absorbs moisture quickly.

Castrol SRF is a hyper-exotic and hyper-expensive brake fluid that is generally used by wealthy Porsche owners at track events. I've seen prices of $78 per liter for this stuff. Sold in metal cans. I can't afford this stuff!