As far as I can tell, the reason oils get more and more expensive is because they are “purer” at the top end of the price range, meaning little to no compression. OK, I could be making this up as I go along here, but it’s how I understand it. I work with extreme long distance hydraulic systems, and we pay thousands and thousands of pounds for a 50 gallon drum of the good stuff (think it’s about £8k off the top of my head) purely because we can be sure that the 6000psi of pressure we apply at our end tranlates to 6000psi of pressure at the business end 30-odd miles away. Use a crappy oil, and our 6000psi might end up only providing 5000psi at the other end, due to the poor compression profile of the cheaper oil.
Translate this to bikes though, you could use the creme de la creme of hydraulic oils in your bike brakes but seeing as you’re using plastic brake hoses (in the majority, anyway) the elasticity of those hoses will negate *any* benefits you have from using the expensive stuff. As long as it’s not eating seals, hoses or calipers, you could probably get away with Crisp ‘n’ Dry.
Probably.