Among the three different types of hydraulic fluid types (mineral based/petroleum based, synthetic esters, naturally occurring vegetable oils) which one is used in the aviation industry and why?
For the aviation industry, flammability is a major concern, and aviation hydraulic systems require Fluids that cannot burn yet don't create corrosion and still lubricate. Skydrol for instance is a phosphate based ester, a nasty substance that irritates the skin, but at least has some flame resistance.
Old hydraulic motion systems on simulators used the stuff, later ones went to mineral oils like Shell Tellus. If there is an oil leak in a pressure hose, a fine mist of fluid droplets appears, flammable if mineral oil but this could be handled in a ground facility with extinguishers everywhere. Not a good circumstance in an aviation accident though, and that is why in aircraft the esters have to be used.
Vegetable oils are not used for hydraulic systems in the aviation industry at this moment as far as I am aware.