Several factors, alone or in concert, can cause engine sludging. The most common is water in the oil, usually from internal condensation resulting from cold startup, or occasionally from a seeping head gasket. This is especially true during short trip winter driving, where the engine doesn't have time to fully normalize and gas off any condensate. At least one auto manufacturer has determined that 10 - 15 miles of freeway operation are required to bring everything up to full operating temperature. Richer fuel mixtures required for cold engine startup combined with high sulphur content fuel can also contribute to sludging. Hence the recommendation for more frequent oil change intervals in the winter, especially for short trip drivers. An often overlooked possible contributing factor is a hot running engine, which can promote oil oxidation and breakdown. So, on the surface it would appear that operator negligence re servicing would be the primary cause of sludge buildup.
However, the engine's positive crankcase ventilation system is designed to purge combustion blowby gases, water vapour, and any other gaseous contaminants from the crankcase. This system should reduce or at least delay the rate of oil contamination. If the PCV system is inoperative or compromised, or poorly designed (inadequate or localized crankcase vacuum), then the rate of sludge buildup could accelerate despite oil changes in accordance with manufacturers' specified intervals. Could engine design be at least partly responsible? Chrysler, for example, currently has an issue with some of their V8 engine oil fillers which don't heat at the same rate as the rest of the engine due in part to placement directly in the cooling fan air stream, allowing condensation to build up and sludge to form in the filler tube and underside of the filler cap.
I'm not sure if this is the answer you were looking for, but it's the collective opinion of 2 other automotive technical professors and myself (I teach emission control systems, among other disciplines), with a current combined trade and research experience of 109 years.
Here is a link woth some good info:
There are other causes of sludge. But I know you work for Toyota and their only response is Lack of mainteance.