I'm no engineer but.....
Liquid is pretty compression-less (note pretty, not entirely), water, oil etc is VERY hard to compress, I doubt tank fluid is any "harder" than water and I doubt you would notice any difference other than the weight saving in your wallet.
Edit: that was just talking about how stiff the brake would be, as has been mentioned water works well because it's thinner than oil, if you really want to experiment you need to try something even thinner, mercury could be a good one