DOT 4 is really adequate enough and there is a higher temp option available should you need it. A new system or a fully refurbished system could arguably benefit from DOT 5 silicone but it costs and is hygroscopic and does need changing.
Rust in rear cylinders. Don't matter if you lube them or not any water absorbed into Dot 3 or 4 will rust the pistons. Oil as you know floats on water so the H2o element will stick to the metal surfaces and cause rusting. This also a-braids the rubber elements and you end up with that black sticky gunk which seizes the pistons solid. Now Dot 5 being silicone chases water out and it is the silicone that sits on the metal surfaces even in the presence of water which does tend to inhibit rusting and the system stays cleaner for longer.
And guess what, they are exactly the same in their action as an hydraulic agent unless you want to boil your brakes where of course silicon has a huge advantage but for a light road going car like the Morgan are you really going to do that. I drive my car quite hard on the rural welsh roads and I don't actually use the brakes much and even when you are they don't feel like I am using them to extremes.
