Actually there should not be any build up at all....If you use a modern oil (API SH or better) you should not experience sludge unless you keep the oil in the engine far too long, do drive exceptionally many short trips or have coolant leaking into the system.
Can you be somewhat more specific about the sludge you noticed (colour, easily wiped away or not, smell etc)? This might give some clue what the reason for its coming into being was. Also the parts of the engine where you found it might indicate a problem.