I use MOTUL DOT 5.1 it's NOT SILICONE brake fluid like DOT 5 is, but, it IS SYNTHETIC.. Synthetic brake fluid has a higher boiling point that regular brake fluid and I am pretty sure that 5.1 is NOT compatible with other synthetic fluids... Usually sunthetic brake fluids are compatible with DOT 3 and 4,, but I remember hearing that 5.1 is NOT.. It's pretty pricey (about 8.00 a pint) but,, it works fine...