A simple Google search of "hydraulic oil" will return an array of oils and fluids from all different manufacturers. So, which one do you choose? Your initial thoughts may be along the lines of "I just need something to put in that will do the job".
This may be the case but never has the phrase 'buy cheap, buy twice' been so true, in fact maybe the right phrase is 'buy cheap, replace twice as often and replace your machine parts twice as quick'. Basic hydraulic fluid will of course do the job, for a limited amount of time. But what does a good quality hydraulic oil do?