Engine sludge happens when the engine oil begins to break down and most vehicles that have engine sludge are those vehicles that are mostly driven in cities and towns and does a lot of stop and go driving.
Stop and go driving and short distance driving can lead to moisture in the oil which can also lead to engine sludge buildup.
Bad crankcase ventilation can also cause the engine oil to break down as well and lead to engine sludge which can then block holes that oil travels through to keep your engine lubricated.
Another common thing that leads to engine oil sludge buildup is not changing oil often enough.
Going more than 10,000 miles or so without changing your engine oil can also lead to the oil engine sludge so make sure you change your engine oil at 10,000 miles or much better at 3,000 to 5,000 miles and change your oil filter as well.