Some general information on hydraulic fluids

The following information was published on the 2CV mail-list on the 9th of august 1995 by Maurice Gunderson

I shall try to shed a little light on the hydraulic fluid picture. I have spent years designing high-performance servohydraulic systems, so I have lots of information at my disposal that most car owners wouldn't have. But all this experience and information leads to a simple conclusion: It's best to use LHM in your CitroŽn!

There are literally hundreds of hydraulic fluids on the market, differing in all sorts of aspects. Many of them were designed for specific tasks, and aren't much good for anything else. To illustrate the point, I have reference materials on special fluids for: The Lockheed SR-71 Blackbird; the Space Shuttle; hydraulically-operated surgical tables; "animatronic" figures such as the animated bears at Disneyland; John Deere tractors; aircraft landing gear shock struts; steel mills; flight simulators; and many more. All these applications are different, and each fluid is different.

There is one common thread, however. The specifications are written by the machinery manufacturers (Lockheed, Disney, John Deere, etc etc), and the fluids are made by fluid manufacturers (Shell, Texaco, Castrol, etc, etc). The machinery manufacturers have the freedom to "design" any fluid they like. Of course, if a standard fluid fits the bill, great! But often that is not the case, and a new set of specifications is generated. Then the fluid manufacturers make the stuff.

Such was the case with LHM and CitroŽn. CitroŽn wrote the LHM specifications to meet the requirements of their hydropneumatic suspension, Citromatic, brake, and steering systems. As I understand the story, CitroŽn worked with Total, the French petrochemical company, to develop the specifications, but after that the specs were available for all oil companies to produce.

The point is that LHM could have been whatever CitroŽn wanted it to be! No previously-existing fluids met the unusual requirements of the automotive environment, so CitroŽn wrote their own. It's just logical to assume that LHM will work the best over the range of conditions that the CitroŽn will be exposed to.

The characteristics of a hydraulic fluid can be summed up as follows:

Base Stock: This is the basic stuff that the fluid is made of, and can be petroleum, synthetic, phosphate ester, silicone, water-based, etc. LHM is made of petroleum base stock, as are the majority of fluids in use today. Base stock determines 90 percent of a fluid's compatibility with other materials such as seals. That is, different base stocks can never be mixed or you get big trouble
Viscosity: this refers to fluid "thickness" or resistance to flow. Viscosity increases with decreasing temperature for all normal hydraulic fluids. It is common to specify viscosity at a standard temperature, usually 100 F. Selection of viscosity is a tradeoff; the fluid must be thin enough to flow freely, and heavy enough to prevent wear and leakage. (Note: To be exact, the "viscosity" I'm speaking of is actually shorthand for "kinematic viscosity," which is measured in Centisokes, abbreviated Cs.)
Viscosity Index: This measures the rate of viscosity change with temperature. The higher the index, the more stable the viscosity will be as temperature changes. Note that fluid A can have higher viscosity than Fluid B (A greater than B) at 100F, but if fluid A has a greater viscosity index than fluid B, then fluid B could have a higher viscosity than fluid A (B greater than A) at a lower temperature. This explains the contradictions as to why the viscosity's of different fluids seem to flip-flop at different temperatures.

The viscosity index of a fluid can be improved by additives, usually polymers. This would seem to "improve" the fluid because it could be used over a wider temperature range, but like everything there are trade-offs. The polymers are destroyed by shearing across valves, which degrades the fluid. So the choice of viscosity index is a trade off between wide temperature range and fluid stability.
Additives: Other additives can be added to the fluid to improve its Rust Resistance (that is, rust of the hydraulic machinery); Oxidation Resistance (that is, oxidation of the base stock); Anti-Foaming; and Demulsification. All of these additives do have the intended results but also have some negative side effects.

And now to answer some questions from cyberhydraulics land:

Q.: Q.: I run my DS (Citromatic) on "red oil." I checked the can. The product I use is manufactured by Texaco and called "Hydraulic Oil 15." Its specification is MIL-H-5606 F. The green fluid I use in my Safari (with 5 speed manual gearbox) is manufactured by Bendix and has the following specification: AFNOR NFR 12-640M. I am guessing the viscosity of the green fluid is 12 weight, compared to the red fluid viscosity of 15 weight. I don't regard this as a slight difference!
A.: Nope, good guess, but the "15" and "12" don't correspond to the same "weight" scale used for engine oil. The SAE Viscosity Grades (10W, 10W-40, 80W, etc) are only meant to apply to engine oils and gear lubricants. This method of rating is not applied to hydraulic fluids. It's just coincidence that LHM is approximately the same viscosity as 10W engine oil at room temperature.

A different rating system, the ISO Viscosity Grade Number, is used for hydraulic fluids. The ISO number corresponds to the approximate viscosity in centistokes at 40 C. Many of the fluid companies (Texaco, Exxon, Kendal, others) have adopted the ISO numbers as part of their brand names, such as "Exxon Teresstic 32", or "Dryden Paradene 22AW." The ISO grade of MIL-H-5606 is approximately 15, so it makes sense that Texaco would call their stuff "Hydraulic Oil 15." Others makers do their own thing, probably just to keep us hydraulics engineers on our slippery toes, such as Mobil, which uses about one-half of the ISO number: "Mobil DTE 10," is really ISO 22.
Q.: I suppose there are several viscosity's offered ? Maurice, is that signified by the letter (A, B, C, etc.) ?
A.: Nope, another good guess, but all MIL-H-5606 is the same viscosity. The suffixes refer to the general revisions of the specifications that have been done over the years. Lots of little details have been changed in the specifications, but not the viscosity. Texaco may make other viscosity's of their Hyraulic Oil, but those won't say MIL-H-5606 on them.
Q.: As an aside, you can run CitroŽn hydraulics on 10 weight NONDETERGENT oil in an emergency. I know at least one person who uses it all of the time in a manual shift car.
A.: Yep, but emphasize "emergency." This is false economy of the first kind. The only thing similar between motor oil and LHM is they're both slippery (well, not quite, they are both made of petroleum base stock). If I wanted to make a CitroŽn malfunction, this would be a good bet.
Q.: On the subject of 'red oil' do you recommend its use ? I notice that you say you use LHM in your DS. A case of voting with your feet ?
A.: No, I sure don't recommend it. The only good things about MIL-H-5606 are that it's widely available and relatively cheap. Not compelling reasons to use it in my opinion. It probably won't ruin anything, because both MIL-H-5606 and LHM are made of petroleum base stock, and that's mostly what effects seals. However, its viscosity and viscosity index are different from LHM and so the car can surely be expected to act strangely at temperature extremes.
Remember that MIL-H-5606 was available world wide many years before CitroŽn wrote the specifications for LHM. If MIL-H-5606 was a good idea, you can bet they would have used it.
Q.: Are there any substitute fluids for LHM ?
A.: Yes, I have some specifications for a couple fluids that would probably be as good as LHM in a CitroŽn, or maybe even better. However these are very specialized and they will be in the $100 to $500 per gallon range. If anybody out there wants to hear about these gems, just let me know!

If you MUST use something other than LHM, I would suggest you use Dexron II. This has a higher viscosity than LHM, and will certainly cause sluggish behavior at low temperatures. However its lubricity is good, and it might serve to lessen leakage in high mileage cars. Its also cheap and its available everywhere.
Q.: What about flushing fluids ?
A.: Well, well! One of my favorite subjects!

Unfortunately, Maurice Gunderson never wrote the answer to this question.

Last updated on: 23.01.1998, by Jint Nijman