jesse farr wrote:

if you fellows really think that using this or that approved brake fluid in something on an experimental aircraft with a rotary engine is going to help you get some sympathy, much less help you find liability where there is most likely none, go ahead, feel better. Remember, I said non certificated aircraft.

Jesse, wasn't worried about insurance or sympathy.  The original question was why is a certain type of fluid used.  The answer was, 'cause the fluid will eat your seals if you use the wrong type.  Certified, approved, insured, etc...I don't care.  I would like the seals to work before I run off the end of the runway, though.

I now have new information, though.  Buna-n is the standard in automotive applications, so any standard automotive fluid should work.  I'll still ask someone more knowledgable than me before I diverge from the 'standard', though.  Standards are a crutch for the ignorant.