Brake fluid refilling - the same DOT is a must ?

[NoHachi]

Thats not correct.

The DOT number is determined by three factors:
1. Composition of the fluid (Only thing you need to know is that DOT5 is glycol based and does not mix with other DOT's. DOT5.1 does through, irritating naming convention that tends to confuse people).
2. Dry boiling point
3. Wet boiling point

Dry boiling point is for fresh fluid. Wet is the boiling point after the fluid has attracted x% of water (as it does in the brake system after a while).
The DOT standards are determined by both a minimum dry and wet boiling point. (see graph: dry is start of line, wet is at x%).
For DOT5.1 these are both higher then the DOT4 standard. Aka, DOT5.1 has a higher minimum dry boiling point AND it degrades less quickly the DOT4. They are however minimum standards.
This means you can have exceptions. For example:
A fluid that degrades very slowly over time might have a wet boilingpoint that meets the DOT5.1 standard. But if it doesn't have a high enough initial dry boilingpoint it is classed as a DOT4 (very rare situation).

Alternatively, some DOT4s have VERY high dry boiling points, but do not meet the DOT5.1 standard for the wet boilingpoint (they degrade quicker). These have to be called DOT4 but are fine for racing, where we sometimes change the fluid each session and don't care about the wet boiling point.
Hope this makes some sense, I'm sure there is enough real info out on the web.

In short: If you are not racing and switching fluid often, DOT5.1 is your best bet and is what you should be using if you care about performance.