Since sludging is purely a function of infrequent oil changes, you can
see the evidence of neglect by removing the oil filler cap and looking
both at the bottom of the cap and into the cam tray. You will see the
evidence of sludge, varish and carbon build up. In order for oil to
sludge it must be hot. Oil temps in excess of 200 degrees F will cause
dino oil to smoke and thicken. On daily drivers this usually occurs long
after its useful life as a lubricant has expired. Since all engines use
some oil, pushing the change interval means that the volume of remaining
oil in the crankcase becomes less and less as time goes on. Since there
is less oil, its contribution to engine cooling becomes less adequate
and the oil runs hotter as the volume decreases, hence the sludge. If
the volume gets low enough there is not enough free flowing lubricant to
protect the bearing surfaces and the engine seizes.

I've seen high mileage (150K miles +) white motors that get regular
synthetic oil changes that look and run as if they were brand new: not a
speck of varnish under the cam cover, no deposits on the backs of the
valves, no grainy carbon junk in the pcv, no lifter noise; just a
perfect example of what regular oil changes with synthetic oil can mean
to the longevity of a motor.

Bob