The recommended fluid for use in your Mini's brake and clutch system is DOT4 brake fluid, which is readily available from most auto parts stores. Please
do not use DOT3 fluid; we've heard from too many customers who've had brake and clutch failures who have also been using DOT3. This does not necessarily mean that the DOT3 caused it, but it's certainly a frequent coincidence.
The manual also recommends that brake fluid be flushed every 24 months. Reason? As time passes, moisture gets absorbed by the brake fluid, which then loses its effectiveness.
Some folks are using DOT5 silicone fluid for their classic Minis, because it does not absorb moisture or damage paint. The problem? It's not recommended for the British hydraulics, and the manufacturers of brake master cylinders
will not warranty their product if silicone fluid is used. If you use silicone fluid you do so at your own risk!