Matador,
All I've ever seen done is to dump the hydraulic fluid into the "waste oil" tank with the gear oil, engine oil, and anything else that remotely might be oil, like varsol and brake fluid.
If you use a lot of diesel fuel on the farm you can investigate cleaning the waste mineral oil and blending it with gasoline to get the viscosity right and then burning the oil in diesel engines. The one caveat you need is to clean the oil with a decent centrifuge. Plain filtering doesn't do it fine enough. You don't have to go through the stage of making biodiesel.
I haven't done it yet but the F350 Ford I recently bought has a 7.3 diesel engine, to be used as a test bench for burning waste motor (mineral) oil. I won't bother trying cooking (veggie) oils, only because, for me, they aren't as easy to get and they can solidify on you 'way easier.
For anyone interested,
here is a basic technical article in how it's done. (The author is a mechanical engineer with a specialty in petroleum, so he knows his stuff.)
Jon.
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