Why do oils become cloudy?

Why is my oil cloudy?
My filter supplier claims that cloudy oil always indicates increased water content.
Is this true?

OELCHECK answers:

In many cases, cloudy oil is an indication of water contamination. The oil becoming cloudy indicates that the water content is above the saturation limit of the oil. The oil becomes cloudy when the water droplets have a size of 2-10 µm. The incident light then reflects off these free water droplets and makes the oil look cloudy. The saturation limit and thus the start of turbidity depends on the type of oil used and the dispersing additives contained in the oil. As additives, dispersants literally envelop small particles of impurities, which also include water. They keep them in suspension and prevent them from settling.

An HLP-46 hydraulic oil often becomes cloudy at a water content of less than 1000 ppm. (1000 ppm corresponds to 0.1 wt.% or 1000 mg/kg).

An HLPD-46 hydraulic oil or a similar to be used engine oil SAE 20W-20 contains special dispersing additives. It can therefore absorb more water and only becomes cloudy at a water content of 0.12 - 0.14 %.

An engine oil such as SAE 10W- 40 or 15W- 40 only functions reliably with an extremely high proportion of detergent and dispersant additives. With these oils, turbidity occurs only in the range of 0.15 - 0.20%.

Gear, circulating and turbine oils, on the other hand, contain only small amounts of additives that can keep dirt or water in suspension. They become cloudy already at a relatively low water content of 0.06 - 0.10%.

In the OELCHECK laboratory, the water content of each oil sample is determined, even if no turbidity is visually apparent. If water is detected, OELCHECK tribologists comment on possible causes of water entering the oil. An important evaluation criterion is the hardness-forming elements such as calcium, magnesium, potassium and sodium. They are only present in tap or waste water, but not in "soft" water such as condensate and rainwater. However, water is not always the cause when oil becomes cloudy. Cloudiness can also result from mixing mineral oil with a glycol-based synthetic oil (or brake fluid). It occurs due to incompatibility of the two product groups. Whether the turbidity is ultimately caused by water or by product incompatibility is precisely clarified in the OELCHECK laboratory with the aid of infrared spectroscopy.