Supposedly DOT 5 will not take on water from moisture in the lines over time. I've always heard that you should flush the system when changing from DOT 5 to DOT3/4. I like the response from maxepr because spilled DOT 3/4 will wreck any paint it touches. I inadvertently switched back to DOT 3/4 without ill effects even though I didn't flush the system. However I drive that car only about 500 miles per year. My conclusion is you can switch, but why?