[bobcowan]

Lots of negatives to the 5, It is generally not recommended for street vehicles. Even Harley quit using it.

5 is silicone based, and all the others are glycol. Silicone is hydrophobic - meaning that it does not absorb water into solution. Which initially sounds good until you realize what that means. Water tends to pool, and is heavier than brake fluid. So the water ends up in the calipers. There's a number of good reasons we don't use water as brake fluid.

Glycol is hydrophyllic, and absorbs water into solution. You won't get the problems with corrosion and steam like you do with silicone. But that's the reason you need to flush it every so often.

Oddly enough, silicone fluid is slightly compressable. You end up with a mushy pedal.

The functional differance between the differant fluids (3,4,5,5.1) is dry and wet boiling points. Get DoT3 much above 350* or so and it turns to live steam. That's a bad thing. 5.1 is fairly new on the market. It has pretty much the same boiling points as 5, but it's glycol.

For street driving, 3 is just fine. If you want a little extra, spring for the 4. I use 4 in my street vehicles, because it makes me feel better.

If you're racing, you need to use some pretty good stuff, and flush often. Motul is pretty popular. I use Wilwood 600 - it's relativly inexpensive and easy to come by.

You CANNOT mix glycol and silicone. I will clot. Most brake systems are built using glycol fluid as a lube and for anti-corrosion. If you want to use silicone based, you need to disassemble everything, clean all the parts, and re-assemble with silicone.

In short, I recommend you sell that silicone DoT5 to some one who owns an old harley, and buy glycol based fluid instead.