Why is my oil tank so dirty on top?
If the tank has been in service for sometime even the slightest drip, spray, or oily part can accumulate over a period of time. Add to that any airborne particulates that settle on the oil and you have a greasy mess. To check if oil is presently leaking on the tank outer surface, clean off any remaining oil and let dry. Tape a sheet of newspaper to the now clean surface and occasionally check for oil leakage. Tanks do have a finite service time and will eventually corrode and need to be replaced. If you are not sure as to the integrity of the tank call your fuel distributor. They should offer a free check-up of the tank.