NEVER!! The oil pan should be removed and clean the sludge out. Some people will drain the oil and replace it with kerosene, diesel or ATF ect.. and ruin their engine doing so. None of these oils should ever be in the crankcase. First they will not lubricate properly and second it will take that sludge into your oil pump and once the oil filter plugs up it will bypass it pumping all that crap into your bearings! Crankcase sludge it must be removed by taking the engine apart and cleaning it. There is no fast or cheap way to do it.
Older engines have sludge for many reasons, here are some of them.. poor quality of older oil, very low coolant temperatures, short trips, fuel additives, no PCV system to draw vapors out.. We strongly recommend adding a PCV system to the earlier engines, it will keep your oil cleaner and most importantly keep you from breathing in the deadly crankcase fumes. Check my tech info on adding a PCV system.