"What is the preferred method for receiving turbine oil — bulk tankers or barrels?"
The "preferred" method will be different from one location to the next and is completely situational, depending on volume, storage considerations, supplier availability and other factors. To make the best decision for your particular application, you should be aware of the risks or concerns of each method.
Among the drawbacks of using drums are that rust and body damage are common. They also can be heavy, weighing approximately 450 pounds on average. In addition, drums are difficult to clean and inspect, and they have a high risk of headspace ingression.
Bulk storage tanks can be fitted with filter panels to continually filter oil and only require one breather, whereas each drum must be fitted with a breather and will need to be filtered individually prior to use. With a bulk storage tank, each delivery eventually will become homogeneous in the storage tank, while with drums, there is the possibility of a drum sitting in storage beyond its shelf life if the first-in/first-out (FIFO) method is not employed.
One of the major drawbacks to bulk storage is the high risk of cross-contamination, as it takes very little to ruin a tank of turbine oil. For example, just one liter of motor oil could have a dramatic negative effect on the demulsibility of more than 20,000 gallons of turbine oil.
Unless you are certain that your supplier is thoroughly cleaning each truck's tank prior to delivery, you may have a problem. Each bulk oil delivery truck has a "heel" or residual amount of oil that is left behind when the truck is emptied. This "heel" can ruin your oil delivery and subsequently your entire on-hand supply.
The bottom line is that there is not necessarily a preferred way to receive turbine oil but rather advantages and disadvantages to consider for each method. Be sure to evaluate each option so you can make the best decision for your particular application.