What's the difference between MIL-H-5606 and MIL-H-83282?
5606 is petroleum based. 83282 is all synthetic.
83828 has a much higher flash point as a result, and is considered 'Fire Resistant'.
That's why 5606 is a backup for 83282... just lacking a few potential features you might want, especially in proximity to hot hydraulic pumps or turbine engines.
Note: Either is highly toxic. It's been a while since I read an MSDS on one, but I believe both are carcinogenic.
Also- ever need some of the stuff and not know how to go about it? The other crewchiefs in my unit always used to say "Hey Sarge, we don't have any of THIS stuff" because they'd look for the part number (NSN) on the can. In reality, part numbers change by container size. What you want is the MILSPEC (MILitary SPECification) to determine whether or not you have the right fluid.
On some commercially labeled products, you might see something like "Complies with MIL-H-5606" on the can. That's how you know that the product you have meet certain production and composition requirements.
DISCLAIMER: Always consult an A&P and appropriate maintenance procedures before selecting fluids for aircraft use.