This "hydraulic myth" is something I have to deal with almost every day, and it resumes to believing that "new oil equals clean oil".
Despite hundreds of sources that state the same thing, a lot of (I would even venture stating that most) people who operate or service hydraulic systems are still convinced that new oil from a sealed container, be it a drum or a jerrycan - is sufficiently clean to be poured directly into the filling cap of the tank.
"We installed a brand new and very expensive variable displacement pump and took all the precautions! We even discarded the "used" oil, and topped up the tank with a brand new one straight from them drums!"
Good for you, my friend! And good for the shop that overhauls your pumps. You just gave them a thousand-hour wear advance! How very generous of you!
The truth is - you never know the state of the oil you're getting. No matter what size container you buy. The transfer/storage/fill processes vary, the dirt does get in - there really isn't much you can do about it. (Especially if you rolled the drums out of the storage area - all the crap that's been lying in the bottom is now suspended again!)
This is why you should treat any new oil as oil with high particulate content - and use a proper filtering cart to transfer it to the tank, preferably thought the system's return filter.