All brake fluid is synthetic. Seriously, go find a natural-based version of brake fluid, it hasn't existed in a very long time. Dot 3, 4, and 5.1 are some variation of polyethylene glycol. That's it. "Synthetic" on the bottle is a marketing gimmick, and all PEG is hygroscopic. DOT 5 is silicone, also synthetic, and is NOT hygroscopic. It is the only one that can't be mixed with any of the others. Switching to 5 requires all new seals and flushing the system completely. It's likely that the above is just someone falling for a marketing gimmick with zero value.