As luck would have it, Road & Track has a mention of this very subject in the Tech Tidbits column.

"DOT-3 brake fluids are glycol-based. DOT-4 fluids contain borate esters as well. DOT 5.1 fluids have advanced chemistry of a similar makeup."
[...]
"DOT-5 brake fluids are silicone-based and are not hygroscopic. Thus, they do not require periodic replacement."
[...]
"But silicone fluids are not without their tradeoffs. Some brake-system components of silicone rubber are attacked by these fluids. What's more, any moisture—and some is inevitable—tends to form globules that seek out a low point in the system. There, these water bubbles can degrade performance even worse than a glycol's absorbed moisture. Last,silicone brake fluids absorb air at a rate three times that of their glycol counterparts. This can lead to a spongy pedal and less than optimal ABS operation.
The two types of brake fluids, glycols and silicones, are utterly incompatible. In fact, DOT-3, -4 or -5.1 mixed with DOT-5 can result in a coagulated mess."

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1989 300 SEL that mostly works, but needs TLC