There is no sludge at all . The crap on the cam plastic cap is due to condensation and oil vapors. The baked on oil on the vanos unit is identification that the vanos is getting very hot , could be due to the fact that the oil is loosing its friction capabilities before its changed or bad seals/springs inside the unit.
But anyway, you should be looking for other things to justify how good previous owner's oil changes were. Look/inspect the cam lobes , pull the cam cap and check for wear. But you need to know what you're looking for .