Synthetic DOT 3 or DOT 4 is fine. All I use is synthetic Dot 4. It is the DOT 5 stuff you don't want to get into. That stuff is silicone fluid and it should only be used in a system that was designed for it. Flush your system with synthetic DOT 4 at least every 2 years and you will be fine. I am always amazed at how bad the fluid looks after just a year.