Don't get the term "
synthetic" mixed up with DOT 5 fluids (which most of us should
not be using). Castrol GT LMA is DOT 3/4, and it's
synthetic, and it is fully mixable with conventional brake fluids. Here's the product data sheet on it and it addresses that issue:
http://www.castrol.com/liveassets/bp...BrakeFluid.pdf