EDIT: OK, what I said is quite wrong or at least misleading, see forum_ghost's comment for discussion of importance of catalytic and enzyme inhibitor/activator activity of such compounds.
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...generally a chemical that doesn't degrade is chemically inert so it has less chances than any other more degradable compounds to cause harm (btw, how much research did we put into the multiple pathways of degradation for the currently fashinable "biodegradable" stuff? how many of those possible intermediary degradation compounds are not in fact quite toxical). (Bio)degradable also means reactive, so things are more complicated than it seems.
But yeah, any non-degradable fluid is probably a very bad idea because it's gonna spill and get everywhere, so even minor probability of toxic effect could have insanely large consequences...