DOT-5 is silicone based. I've always been under the impression one can not mix dot-5 in a dot 3 or 4 system.
Like you all said, the seals and stuff will leak. Dot 3 and Dot 4 are compatable.
My opinion, stick to 3 or 4 and just flush the system every few years. It really isn't that difficult.
Jay
