From wikipedia article
https://en.wikipedia.org/wiki/Brake_fluid Dry boiling point Wet boiling point Viscosity limit Primary constituent
DOT 2 190 ?C (374 ?F) 140 ?C (284 ?F) ? Castor oil/alcohol
DOT 3 205 ?C (401 ?F) 140 ?C (284 ?F) 1500 mm2/s Glycol Ether
DOT 4 230 ?C (446 ?F) 155 ?C (311 ?F) 1800 mm2/s Glycol Ether/Borate Ester
LHM+ 249 ?C (480 ?F) 249 ?C (480 ?F) 1200 mm2/s [6] Mineral Oil
DOT 5 260 ?C (500 ?F) 180 ?C (356 ?F) 900 mm2/s Silicone
DOT 5.1 260 ?C (500 ?F) 180 ?C (356 ?F) 900 mm2/s Glycol Ether/Borate Ester
Wet boiling point defined as 3.7% water by volume.
The different viscosity and major components can affect how brake systems work. The LHM+ is the citroen style fluid.
Current standard components for an Elan appear to work Ok with DOT 3 / 4 / 5.1
Use of DOT 5 has been known to give problems in some cars and be Ok in others
I used it for a few years in my PLus 2 but i did not like the soft pedal and the brake booster tended to have a slow action and then come in suddenly so i changed back to DOT 4 which rectified these problems
I run DOT 5.1 in my Elan with no problems but its cost is much higher and only really warranted or needed on a track car.
Most cars that specify DOT 3 can also use DOT 4 or 5.1 but not all. My Toyota Landcruiser specifies DOT 3 and I used DOT 4 once for a routine change as that was what I had in the garage for the Lotus and I thought it should be OK. I found that the brake pedal started sticking and the antilock brake system warning light was coming on. I changed back to DOT 3 as specified and all the issues went away. Maybe a viscosity issue or a seal compatibility issues with the different composition of DOT 3 to 4.
While silicone DOT 5 fluid is not hydroscopic like the other fluids, water still gets into the wheel brake calipers overtime as it gets passed the seals as the pistons move with ever brake application in miscrosopic amounts and it sits as free water in the caliper cylinders and causes corrosion. Only really a problem if you drive your Elan in the rain but it can happen.
cheers
Rohan