If you have proper 'access' ports on the tank ....
Drain tank and take old oil home to burn in your oil heater.
Long handled scrub brush to knock the agglomerated particulate from the tank walls ..... sop up residue from tank bottom with paper towels, then burn !!!!!!
Then pressure washer, and pump the water/oil waste to a 55 gallon drum and let it gravimetrically stratify .... soak-up the oil that eventually floats on top of the oil/water mix with paper towel s (let the towels 'dry' then burn the towels) or use 'bilge boom' absorbant pillows (polypropylene fibers/matting rolled up into 'log) & discard at a recycle center (or burn).
Then ..... when the tank is visually clean, add a few gallons of NEW fuel and recirculate through a polishing filter @ 1uM rating (many times through the SAME filter).
If you dont mechanically remove the 'crud' from the tank walls first, all the subequent filtering/polishing will be very inefficient. You have to get the 'crud' off the tank walls FIRST.
Hope this helps.