Both my trucks are DOT 3 "regular" or were until I was in need of fluid and I found that DOT 3 Synthetic had taken over the auto parts store shelf and they weren't stocking regular any more. So, with a frown, I used the new DOT 3 Synthetic to top off my '37. It is a daily driver and that was over a year ago. I have not noticed any problems.
As far as disappearing fluid, you might want to pull your drums to see if there are any wheel cylinder leaks.
