If it was sold as "food grade", you may be able to at least verify that with the markings on the drum. Manufacturer, material, warnings, etc.
These days, anything that comes in contact with skin or gets ingested is gonna be stored in food-grade containers; so that I wouldn't worry too much about stuff embedding itself in the plastic. Getting oily / sticky stuff off of it may end up being a bigger test of patience than you may have bargained for and just a big PITA.
I thought about buying a used drum like this long and hard. Honestly, your story's been told numerous times. Enough times that I decided I'd buy a new drum or keep thinking. I ended up finding a used 45G that fit in a space much better than where I was planning on putting the drum, so that worked better for me and MUCH cheaper. Depending on your situation the Petco sale makes 29's and 40's a bargain compared to new drums and zero work compared to used drums.
"Why can't my wife see all of this stuff as an investment?"