First, why are you considering DOT 5 brake fluid? This brake fluid is formulated with silicone and is not recommended for use with ABS systems. Also, silicone-based fluids should be mixed with glycol-based fluids....they will coagulate.
I'm not sure what is recommended in the owners manual for your 1991 model, but you may be thinking of DOT 5.1, which is a glycol-based brake fluid (DOT 3 & 4 are also glycol-based), and has superior dry and wet boiling points.
95 E320 Cabriolet, 131K