Exceptions can be made based on specific needs or situation. A separator's efficiency is most affected by settled solids or sludge and by oils. Any resistance to push through to the bottom will indicate a sludge buildup. Measure oil floating on top of the water. When there are two inches or more of oil in any chamber, it should be removed.
Older oil has achance of becoming emulsified.
These firms have special vacuum trucks that pump out materials with the consistency of anything from liquid slurry to solid dirt. The bulk liquid is shipped to a licensed treatment facility where the oils, solids and heavy metals are separated from the water. The treated water can be discharged to the sewer. Choose a reliable vendor by making sure its equipment is right for your situation. Some equipment requires sludge to be a pumpable slurry.