The process of cold-cracking (which means you chill and centrifuge the oil to stop it from distilling) means sludge develops in your oil tank. This is due to the shedding of sludge during this process of making the oil clear. It must be dealt with because oil tank sludge can clog filters and pipes, which consequentially reduces heat produced by oil heating units -- it could even cause them to fail completely in the worst scenarios. Removing sludge is simply essential in keeping your oil tank in working order.
Add an oil-heating additive. You will be able to purchase bottles of this in most large hardware stores, or you could choose to shop online. Heating oil additives can break up sludge in the tank and make things run smoothly again. Unfortunately, this may not fix clogs in the pipes and filters.
Allow the sludge to settle in the tank by turning the oil tank off for a few hours. Although this does not remove the sludge, it can, at least, aid the flow of the oil again and help your tank run at close-to-normal efficiency levels.
Talk to an expert who will blow the oil lines out. This is a fairly dangerous procedure, so it should be done by an expert. If this does not work then you may have to replace your oil tank. This is fairly expensive, but if none of the other methods up until this point have worked then it may be necessary.