I have been looking through some web sites concerning carcinogens in oils that would be shipped to a fuel oil/lubricant dealer and it appears to me that a barrel that was used to deliver hydraulic oil would be relatively safe. In most cases, hydraulic oils do not contain much if any carcinogenic materials. AS the oil is not used as a lubricant many of the additives that are in motor oils are not present. I checked out safety sites from AmsOil, Mobil, etc and found that carcinogen is not listed as a hazard in their hydraulic fluid.
Of course it is best to be very cautious in selecting the barrel and the oil dealer is required to carry the safety sheets of any of their oil products and should be checked before purchasing a cheap barrel. Food barrels would always be the safest to use as they cannot be used more than one time before being disposed of but lately, most food quality barrels are now made out of recyclable plastic and a steel barrel may become difficult to find in the future.
In any case, it is very important to always check to see what the barrel was used for before making a decision to use it to smoke your food. Burning out the barrel is always necessary no matter what sort of barrel you find.