It's almost impossible to beat a hot tank cleaning job, but the solution in the tank has to be reasonably clean. Most if not all shops push it too far before they change it out or rejuvenate it. Short of hot tanking it, and depending on how sludged up it is you can do a fairly good job with oven cleaner, paint stripper and other solvents. Pull all oil galley plugs out and procure some brushes normally associated with gun cleaning to get into the oil galleys. After you have done all that then take it down to the local car wash and give it a good washing. DO NOT however take a heavily sludged up block right down to the car wash and commence to blow that cr@# all over the car wash. That will get you banned from the car wash quicker than a muddy 4 wheel drive will.