I agree. I don't see any reason why you can't use the DOT 5.1 fluid in your brake system.
Not to mince words, but I believe both DOT 5 (silicone) and DOT 5.1 (glycol) could be called synthetic. So be careful when ordering, to refer to the DOT number and not refer to it as synthetic or non-synthetic.
95 E320 Cabriolet, 131K