Silicone fluid (DOT 5) is synthetic, but that does not mean that all synthetics are silicone. Never use DOT 5 in any DOT 3/4 application. I've seen motorcycle brakes that call for DOT 5, but never on a car.

GT-LMA has been on the market for decades (and I've used it in many cars), but I do not recall seeing "synthetic" on the label. What's "natural" about DOT 3/4 anyway?