I have quite a bit of research on this for work. As far as viscosity goes I have never seen any mention as long as you are dealing with common hydraulic fluids.
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Vickers Mobile Hydraulics Manual, page 372, section: Heat or thermal contamination.
Hydraulic fluids are typically not considered 'high temperature' fluids. Their useful life, as well as maintenance of viscosity, chemistry and the other attributes discussed in Chapter 16, is based upon continuous operation below a critical temperature. This critical temperature is 140°F. Every 18°F increment higher than 140°F effectively doubles the oxidation rate of the hydraulic fluid (petroleum based) thus cutting its useful life in half, e.g. running a system at a consistent 176°F would reduce the useful life of the fluid by 75%.
Most fluid manufacturers specify optimum ranges of temperature for their products - typically 90-120°F or 100-130°F - even though many fluids are operated in excess of these temperature ranges. The effects of temperature on the fluid are the same, regardless of the source of heat.
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What is Oxidation in Lubricating Oil?, by Mark Barnes, NORIA
For most mineral oils, a general rule of thumb is that the rate of oxidation doubles for every 10°C (18°F) rise in temperature above 75°C (165°F). Because of this, synthetic oils are often required in high temperature applications to prevent rapid oil oxidation.
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Oil Hydraulic Systems, by S.R. Majumdar
2.11.3 Oxidation
A dominant factor which determines the service life of the oil is degradation through oxidation. Oxidation occurs when oxygen combines with the original hydrocarbon molecules angives a chain reaction. Traces (even a few PPM) of certain metals, particularly copper, iron, zinc, lead and water act as catalysts and markedly increase the rate of oxidation.
Temperature is another factor in accelerating the rate of oxidation. As a working rule oxidation rate is doubled for each 10°C/18°F rise in the temperature. An oxidation-inhibited high viscosity index (HVI) oil may give a useful life of ~100,000 hours at 40°C/104°F even under the condition of aeration and catalysis. When the oil temperature is raised to 60°C/140°F, life drops to as low as 10,000 hours.
Oxidation produces soluble as well as insoluble products and these may form sticky substances and may deposit as gum or sludge in oil passages, various pump parts, valve spools and ports, etc. blocking them and restricting oil flow and making the machine run sluggishly. As a result of oxidation, the oil becomes progressively darker and gets acidic properties which ultimately deteriorates the life of the oil.
2.11.4 Total Acidity Number
Total acidity number (TAN) or the neutralization number is the measure of acidity of oil. It is defined as the number of milligrams of potassium hydroxide required to neutralize one gram of oil (mg KOH/g). Oil enriched with anti-oxidant additives has a long induction period. Such an oil takes a longer period to reach the TAN value of 1, but after that the increase in TAN value is very sharp and it quickly reaches the value of 2. Though the permissible value of TAN is 2, a sharp increase in value is a better indicator for changing the oil. Once the sharp increase is noticed, it is a warning to the plant engineer to prepare the machine for shut down for changing the oil.
