I could be wrong, but I don't think you are supposed to use synthetic brake fluid in the Teves II system. I think it's because the synthetic fluid doesn't swell the seals/o-rings like regular DOT3 and therefore leaks could develope. I'm sure you can find more info by doing a search on this site.
The way I look at it anyway is, if it ain't broke, don't fix it. DOT3 has been working great for me and as problematic as this brake system can be, I don't need to throw another possible problem into the works.