I assume by synthetic brake fluid you are refering to DOT5 "silicon"?

While I am a great fan of this in my old non ABS Corvair and Porsche it is not compatible with ABS systems. Too bad because I'm still using the same wheel cylinders and master on the Corvair that I rebuilt/installed almost 15 years ago! One problem with the stuff is that it traps air and doesn't wet the metal enough to get small air bubbles completely out of the system. Also, while the DOT5 silicon is "compatible" with DOT3 and DOT4 it doesn't mix with it and is lighter so the old DOT3/4 stuff will set in the bottom of cylinders and won't flush out. The only way to get the benifit of this stuff is to COMPLETELY disassemble, clean and reassemble using the DOT5 and even then bleeding is still problematic.

Most of the ABS manuals say they are not compatible with synthetic and the last bottle of synthetic brake fluid I got says on the bottle that it is not for use with ABS!

Too bad they havent found a better brake fluid material. Just flush the stuff every two years or sooner. Using this approach, my old '85 diesel was still running with the original wheel cylinders when I sold it. (The booster had to be replaced though.)
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'96 965, 16' wheels, rear 18mm bar & Koni + 204HP cams at 123K. Had '85 745 Turbo Diesel for 200K.