I contend the sludge came from the engine seeing many "short trips" around town and too long of time between oil changes. The "short trips" do not allow the motor to heat up enough for a long enough time to sucessfully evaporate the condensation from the crankcase etc. Thats why the mfg have certain milage AND certain TIME requirements for oil change intervals, the milage requirement is apparent-but the time requirement seems silly. It is the time requirement that tries to take care of the condensation laden oil. Blowby contains elements of combustion particles including fuel, these would mostly be caught by the oil filter. Condensation tends to want to cling to cooler surfaces (good reason to run a warmer thermostat). If the pan had a lot of sludge, and if you really want to be sidetracked some more, try checking the valve cover and side plates, both good cooler surfaces for sludge to cling.
