Check Brake Fluid on Wikipedia.
DOT 5 is not interchangeable or compatible with DOT 3, 4, and 5.1 fluids and can cause catastrophic system failure.
Dot 3, 4, and 5.1 are glycol ether based. They are compatible, but like motor oils, you should use the recommended or higher grade fluid.
Dot 4 and 5.1 also have borate ester to handle higher temperatures.
DOT 5 is silicone based and is no way compatible.
Dot 5.1 is sort of OK but not recommended to mix with 3 and 4.
To use Dot 5, you MUST replace all rubbers, seals and flex brake lines as well as flush steel lines.
How do I know ? I am running it in my muscle car.
EDIT EDIT of my not being able to read:
From the 96 Civic owners manual: "use Genuine
Honda Brake Fluid or an equivalent
from a sealed container that is
marked DOT3 or DOT4 only. Brake
fluid marked DOT5 is not compatible
with your car's braking system...
the same DOT3 or DUT4 brake fluid
specified for the brake system [for the clutch]."
You can absolutely use DOT4 in place of DOT3 brake fluid. The only thing you'll need to do is completely purge the system of the old DOT3 fluid to take advantage of the DOT4 features (higher boiling point both dry/wet). The only real difference between the two is how hot they can get before they boil. The DOT4 will not harm your DOT3 system in any way. Do ...
Basically, there is no difference as far as what you can use. Put away all of the hype language on the bottle and focus on the DOT3. This is the main standard you need to look at. (Here's some easy brake fluid reading for you.)
If your vehicle calls for DOT3, you should be able to use any one of the three you have listed. If it required DOT4, DOT5, or DOT5.1,...
The Bosch Automotive Handbook confirms that mineral-oil-based fluids are not hygroscopic:
Mineral-oil fluids (ISO 7308)
The great advantage of mineral-oil-based fluids is the fact that they
are not hygroscopic, so the boiling point does not drop due to
It also makes clear that mineral-oil fluids are incompatible with systems designed ...
The only difference between the two is the boiling point of the fluid, with DOT4 having a higher range. You could use either and get away with it, but if the cap on the master cylinder states it should be DOT4, that's what I'd go with. Even if DOT3 is all that is required, DOT4 will add a little bit better boil resistance. The two different types can be ...
A couple of things here. First, when I find the brake reservoir low, I will look at my disk brakes to ensure the pads aren't shot (or nearly worn out). Usually, if worn enough, the fluid level will drop to a point where the brake light will come on as you described (most notably so if both back and front are worn out).
With that in mind, consider just ...
Dot 4 supercedes Dot 3.
If you put Dot 4 in, no problem. Basically, Dot 4 can go in and replace Dot 3, chances are you already have Dot 3 in anyway.
Wikipedia, Dot 4:
While a vehicle that uses DOT 3 may also use DOT 4 or 5.1
Note: Dot 5 is not compatible with either, it is a different chemical makeup based on silcon. Putting Dot 5 in a Dot 3/4 system ...