VARNISH!
This simple word is on every lubrication engineer's mind these days. It plugs up passages, gums up sliding parts, and generally makes a mess.
Varnish is actually very tiny soft particulate that is caused by breakdown of the oil. Water seems to be one of the culprits in forming varnish.
Varnish can actually be filtered out of a lubricant by use of special filters that have a special molecular attraction for the varnish. Electrostatic precipitators are also being used.

Varnish is deposited at lower temperatures. Higher temperatures keep the varnish moving with the oil.

At my work we are having problems with varnish causing hydraulic valves to stick. When the spring is supposed to push the shuttle in one direction the shuttle doesn't move and the hydraulic system doesn't work. If clearances are small, it doesn't take a very thick film to stop the valve.

R.