if you touch the sludge and its semi solid and sticky then I would suggest you clean this mechanically first, with brushes and solvent and lift out any solid sludge - let the solvent drain into the engine - no problem here because you would be removing the sump too.
once its clean from the top you need to undo the sump and clean it out, you will also need to remove the oil pump pickup strainer and clean it or replace it. pack up the sump and valve cover, remove oil pressure switch and install a pressure guage in the hole, fill with engine oil - but leave out 1/2 litre, fill in some kerosene in place of that half liter and start the engine (dont forget a new oil filter)
start it and let it idle till it warms up, do no rev the engine in any case. watch the oil pressure gauge closely. We do not want the engine to lose oil pressure in any case.
drain out the engine oil after the warm up (usually 20 minutes) and fill with fresh engine oil and new oil filter.