"What could cause a monograde engine oil used in a winch to become pasty with a thixotropic behavior? The used oil almost freezes at ambient temperature (25 degrees C) with water contamination of 0.4 percent."

By definition, thixotropic describes a fluid or gel that is viscous in static conditions but experiences a decrease in viscosity once stressed, making the fluid or gel flow more freely. Non-Newtonian fluids perform similarly to thixotropic fluids but with varying effects. These fluids may increase or decrease in viscosity based on the shear rate. Newtonian fluids have more of a linear change based on the stressor.  

Most oils relate closely to a Newtonian fluid. Monograde engine oils are good at staying linear because of not having an additional viscosity index (VI) improver. With the addition of a VI improver, shearing will take its toll on the additive over time and start to reduce the oil's viscosity.

In typical winch applications, you primarily are looking to lubricate bushings, bearings and gears. Most winches require an SAE 30 or ISO 100 oil. An engine oil like a monograde SAE 30 enables quick lubrication to all internal components. The problem with using a monograde SAE 30 in a winch application is the environmental factors. The temperature will have a major influence on how the oil performs in colder and warmer environments. Water can also have a huge impact on the oil.

While the oil and winch in question only have 0.4 percent water reported, this is extremely contaminated. It is equivalent to 4,000 parts per million (ppm), as every 1 percent equates to 10,000 ppm. Depending on the application's criticality, the moisture content should be in the range of 500 to 1,000 ppm or 0.05 to 0.1 percent.

With engine oils, it is important to monitor moisture content in certain applications. Most engine oils are formulated without demulsifiers to help manage water that enters the system. In automotive applications, the engine temperatures will flash water off, preventing it from forming free water or becoming emulsified. Emulsified water has the appearance of a milkshake and is very damaging.

If elevated moisture content breaks down the additives, the oil will form sludge. This is what you are referring to as the oil behaving in a thixotropic manner and freezing at ambient temperatures. In this case, it seems an oil change is long overdue, as the oil is at the point of having its chemical and physical properties being changed. The best advice would be to drain the old oil and perform a system flush, removing all sludge and varnish that has accumulated. Also, be sure to inspect all internal components for damage and replace them if necessary.

After the system has been properly decontaminated and fixed, you must address how the moisture is entering the reservoir and determine how to keep it out. Desiccant breathers are ideal for applications with moisture contamination problems. Finally, make sure the oil is changed on a time-based or condition-based interval. If using condition-based intervals, set targets according to the cautionary and critical limits.