The MSDS for 5606H lists it as

Distillates (petroleum), hydrotreated light naphthenic

Whatever that is it sounds nasty.

The MSDS for ATF lists it as

highly refined mineral oil

I'd probably wash my hands in ATF without worrying.

DOT-3 is a mixture of Glycol and Glycol Ethers. Not alcohol. Don't breathe the fumes unless you want to go to sleep, permanently.

DOT-5 is Polydimethylsiloxane... Huh???? Some art of silicone. Unpronounceable but it doesn't kill rats.

As for substituting one fluid for another by definition we are experimental aviators. But that's not the real issue here. Neither is cost.

The real issue is availability. So you fly into a small GA airport and your brake goes flat due to a slow leak and lack of fluid. You ask around and most line boys will give you a blank stare when you ask for "MIL SPEC 5606H"

Last I checked there were only two outlets for Aircraft Spruce and they are not open on Sundays.

Scour your airport after 6 PM on a weekend and see if you can find 5606, and really, I don't know of anyone that carries a spare pint of hydraulic fluid in their airplane.

But a trip to the local Wal-Mart will be successful in a search for "brake fluid". DOT 3 is everywhere. DOT 5 is a bit hard to find. My local Autozone and Oreilly's do not carry it, but the New Holland tractor dealer does.

Both DOT-3 and -5 have some nasty habits of eating some seals and paint. ATF is easy on paint and seals but I have no clue as to how it resists the high temps in brake pucks. My Case 480 backhoe uses general purpose hydraulic oil for the brakes and a placard warns against using DOT -3. go figure...

I think the simple test is to put some chosen fluid into a jar with some old brake seals and set it on the shelf for a month. What the hey... You may find that Wesson oil is the answer.

John