The following information was published on the 2CV mail-list on the 3th of august 1995 by Maurice Gunderson who drives/ownes a 1971 DS-21 Pallas (with genuine LHM in its veins).
Allow me to add a few comments based on my
previous life as a servohydraulics engineer.
The aircraft hydraulic fluid being discussed
here is MIL-H-5606. Sometimes there will be an additional suffix,
such as -A, -B, or "micronic," but for CitroŽn
purposes all MIL-H-5606 is the same thing.
MIL-H-5606 is a standard mineral-based hydraulic fluid that has been around for decades. As the nomenclature indicates, the specification was originally developed by the US Military, but it is used in many civil and military aircraft ranging from single engine light planes to jet airliners. It is available at just about every airport in the world, and can be purchased in everything from one-gallon cans to 55-gallon drums. In the aircraft world it is casually referred to as "red oil," just like CitroŽn people refer to LHM as "green fluid." It is marketed by many petroleum companies, and Aeroshell is just Shell's brand name for it. Texaco, Chevron, Castrol, etc. etc. all make the same stuff.
I have the complete engineering specifications for MIL-H-5606, but I have never been able to get hold of the complete specifications for LHM. However, near as I can tell, they are just about the same thing except that MIL-H-5606 has a slightly lower viscosity than LHM. I can see no reason why MIL-H-5606 shouldn't work fine in a CitroŽn, except in the case of a high mileage car where the lower viscosity might cause higher internal leakage than you would get with LHM.
And, of course, "red oil" is red and "green fluid" is green! But that is just a matter of the dye they put in them for identification purposes, and won't have anything to do with performance. They are both made of petroleum stock, and if they left the dyes out they would both be a very light straw color.
While MIL-H-5606 is the most common aircraft hydraulic fluid, it is by no means the only one. Many other aircraft fluids (phosphate ester, Skydrol, etc.) ARE NOT compatible with LHM, and would RUIN you car instantly if not faster! If you want to buy aircraft fluid for your CitroŽn, make sure the label includes the MIL-H-5606 indication, no matter who makes it or what the rest of the label says it's good for.