I highly recommend engine flushes. I ran one on our old maxima which had a good chunk of sludge and i got 9 liters of fluid out. (the VQ engine is about 3.0L for reference - so that means that the engine flush fluid ate up and cleaned up the engine to produce 6L of sludge, which was all removed from the car!) Car ran like a gem afterwards, and this was at about 197K. (Maxima engines are the sh!*T!