The OEM fluid is DOT 3. If your going to clean and flush the system then DOT 5 fluid would be the perferred fluid. It is resistant to not only higher temps but won't absorb moisture either. Either DOT 4 or 5 should not be mixed with any other type of fluid.
I used DOT 5, (silicone) fluid in a 69 GTX and in 10 years and two lining replacements never had a hydraulic issue. It costs a bit more then 3 or 4.