Sludge builds up because
1 you using the oil beyond it's due change time (most people are from this category)
2 your engine for some reason is running pretty hot thus cooking up the engine oil (almost all cars only have a oil light to indicate pressure but usually almost none of them have a engine oil temperature gauge if you have and know how to read and respond to it then you can avert many situations)
3 you using the oil which is low in rating for the required car's application...
If you dislodge that not all of it will end up out while draining some will be there to stick to the oil strainer and some might actually be sticking in those vain like oil lines which will later on invite trouble....