I say Ron is right.
I tried dot 5 in my 67 GTX. With all new wheel cylinders and master cylinder flushed system. As directed. Refilled with silicone dot 5. 3 years later i was out for a drive came up on a curve my Brake pedal went to the floor? I took the GTX to the garage and I was dumbfounded. The wheel cylinder was eat up with rust the fluid was like brown jelly? Was it bad fluid or what .i have found no answers yet
.replacing all cylinders and using Dot 4 flush system every 2 years