Like tankie said, taking the unit to experts would be the most recomended.
One of my units was looking so bad with oil and dust, that I figured the acid stuff and bacterias in there would soon eat the copper wires and decided to take the risk and did this:
Immersed the unit in a good quality refined kerosene and brushed it with a fine painting brush to remove the oil. The unit looked clean but I didn't want that oily kerosene in it, so I took the risk once again and immersed it in water with neutral detergent
and brushed it gently till clean. Rinsed in water 
, hang it under the sun and air sprayed the unit till dry. May sound crazy but it is now clean, free of oil, bacterias and acid. Tested the unit and every reading was fine and it is fine to present. Not that I recommend anyone to do this, but in my personal opinion it is better having a clean unit, that one filled with corrosive dirt media.