Hi George...haven't seen you in a while!
You don't need any detergent to remove sludge in the bottom of the tank, just water...a couple gallons into the tank via the deck pumpout--'cuz that sends the water into the tank at the bottom to stir up any sludge and hold it in suspension so it can be pumped out...pump that out...repeat...repeat...till you're pumping clean water.
However, if the sludge has hardened into "concrete" don't wast the effort...'cuz nothing will dissolve it that won't also damage the system.
This is a job that should be done every couple of months, especially in preparation for winterizing...why are you doing it now?
If you feel some need to clean the oil scum off the tank walls, use only LIQUID detergents, without bleach...low suds. You'll need a gallon, maybe even two depending on tank size to accomplish much. Pour it down the toilet, fill the tank...then go sailing and tack a lot. Leave the solution in the tank for a day or two...even till next weekend won't hurt. Go sailing and tack a lot again. Now dump or pump the tank, nominally rinse it out...you're good to go.
