The correct answer is C) they are essentially self-priming and capable of a high suction lift. Positive displacement, helical gear pumps are well suited for pumping oil because their design allows them to be self-priming and have a high suction lift. This means they can pull oil from the source and pump it effectively, even if there is some distance or elevation change between the pump and the oil supply. The other answer choices, while potentially relevant to other pump types, do not directly address the key advantages of helical gear pumps for oil pumping applications.