Just studied brake fluids, so here's some knowledge I gained:

DOT stands for department of transport, but the relevant spec is ISO 4925, which defines classes. Everyone uses DOT numbers anyways. All except 5 but including 5.1 are glycol cocktails. DOT 5 is silicone and completely incompatible, don't mix. Some Citroens use mineral oil (LHM).

The difference are boiling point and viscosity. If you have ABS/ESP, you want a low viscosity fluid.

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