As asked above, how old is it.
As an owner of a buisiness you are liable if you get caught disposing of anything contaminated with PCBs. The legal and environmentally correct thing to do is call a local environmental waste company (we use Safety Kleen local to us) and have it tested. If you dispose of it the same as you would normal motor oil and it is traced back to you you will be liable not only for the disposal of the original oil drum you contaminated but all the oil you contaminated in the process. This could be as much as an entire tanker truck depending on how far it gets.
And don't think they can't trace it back to you, if you are disposing of your oil with any liscenesed company they track every customers oil with a batch number. I have heard stories of large companies paying hundreds of thousands of dollars in dispoal fees and fines when they could of simply properly disposed of the 15 or 20 gallons and been done with it. I'm sure lots of companies get away with it, but the repercussions just aren't worth it IMO.