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Here's how I would say it (I welcome corrections)

Coking occurs at the highest temperatures.
Varnish occurs at medium temperatures.
Sludging occurs at even lower temperatures.

An example of coking is hard black deposits on the oil-immersed metal reversing contacts of a transformer load tap changer. I'm guessing temperatures 500F+. Copper melts at 2000F.

An example of varnishing - rolling bearing found with brown stains. Perhaps 200F-400F?

Example of sludging - goop that settles from really old oxidized oils and may clog orifices or coolers.
Last edited by Registered Member

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