Synthetic and natural brake fluids DO NOT MIX!
You must remove all of the fluid (it blow out all the lines and master cylinder and then clean with brake cleaner) prior to changing to a DOT 5 (synthetic) fluid. There are some synthetic blends that say they are compatible with regular fluid...but I would be skeptical.
I run ATE Super Blue and have been happy with its performance.
Good Luck.
Nick.
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Nick Shumaker
1982 911SC Coupe
nickshu@yahoo.com
PCA -- Rocky Mtn. Region