I have heard when you but in bulk you can get Rotella in the $8-9 a gallon range. I am not sure how much volume is required. I may use 200 gallons a year. Compared to 12-13 a gallon at stores that would be $800 savings.
For me buying in 5 quart jugs IS bulk buying. I can store those up out of the way on a shelf. A 55 gallon drum would just be in the way and I don't use that much oil to begin with. Having to buy a horizontal drum storage (much better than a pump) rack, keep up with containers and keep them clean would be just too much trouble for the amount of oil that I use. Also as posted, sometimes it cost more in bulk than in gallon containers ( you do have to pay for the drum also) and just like in Sam's Club, you have to watch some of the bulk buys as they cost more per oz. to buy than small containers.
I suppose if I were like Diamondpilot and serviced 100+ vehicles and used a lot of oil, I might look at striking a deal with a distributor to see if it were advantageous price wise but it would have to be more than $1-2 per gallon discount to make up for the inconvenience.
Even if servicing 100 vehicles, if you have to go to the vehicle, hauling a 55 gallon drum of oil with you would be a PIA compared to gallon jugs. Really the only way I could see it being practical is if you have a Jiffylube or similar shop so everything comes to you and you can set up a permanent station with calibrated pump etc so you don't have to handle any containers nor dispose of hundreds of plastic containers per day. I think the drums are returnable also or maybe one could get it in the 275 gallon TOTE tanks for really big savings.