Unless it has problems resulting from the apparent sludge (loss of compression or oil pressure, chronic turbo failures, etc), if the engine is healthy otherwise you can probably take some steps to clean it up without taking it apart and save yourself a lot of work and hassle.
A motor oil lab analysis (Blackstone, etc) would also be a good idea if you are concerned about the contents of the oil. Not expensive and it will tell you about the presence of wear metals or any contaminants like coolant or fuel.
If everything checks out well and you just want to get the inside of the engine cleaned up a little, doing a crankcase flush with diesel fuel will get you there, though it's a little tedious to do the repeated oil changes to get the fuel out afterwards.
Tell us a little more about the car and its history, sounds like you have something interesting with a 1988 740 TD. Is it a converted gas car or imported from overseas? Welcome to the forum.
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86 745 D24T/ZF 345k lifted 2.5"
83 764 D24T/M46 155k
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