They're not carrying OIL. They are carrying REFINED OIL PRODUCTS (gasoline, diesel, and similar) which the Venezuelans can no longer produce for themselves, because they have screwed up their own refineries so badly and driven off the educated class that once ran and maintained them.
Hugo Chavez fired all the competent people who were working in the refineries because their loyalty to the regime was questioned. They were replaced by unqualified political flunkies. Not just engineers and managers, but also technicians and operators were fired. I know some of them. One engineer managed to get out and landed on her feet up in Fort McMurray, Alberta. She puts up with winter, but she’s happy to be away from the Chavista mess.