I don't want to seem contradictory, Mark, but the Lucas DOT 5.1 I'm using is synthetic, although I believe there are non-synthetic fluids that also meet DOT 5.1. There are even synthetic DOT 4 fluids available. What is to be avoided is DOT 5, if you can find it at all in the UK, as it is silicone-based and will not mix safely with even the tiniest amount of DOT 3/4/5.1 in your system. I have heard people say it will damage the seals, but I have never seen anyone recommend a change in the type of seal fitted when changing from a glycol-based fluid to a silicone-based fluid, although scrupulous cleaning (i.e. dismantling and cleaning everything with alcohol) is essential to remove all traces of the old fluid. Silicone fluid will coagulate in the presence of glycol fluid.
Anyway, back to the plot. DOT 5.1 is the highest spec. commonly available but there are DOT 5.1 fluids that exceed the spec. by more than others. 5.1 is designed to be less viscous than DOT 4 to help brake modulation in ABS systems.
ATE do a blue racing fluid which I believe will withstand temperatures much higher than will be experienced on the road but I'm told it is very important to change racing fluids very frequently, like after every race.
Anyway, I'd say use a good name-brand DOT 5.1.