The correct answer is D) they are essentially self-priming and capable of a high suction lift. Positive displacement, helical gear pumps are well suited for pumping oil because they are self-priming and can generate a high suction lift. This allows them to efficiently draw oil from storage tanks or other sources without the need for additional priming or suction devices. The helical gear design creates a continuous flow of the pumped fluid, making these pumps ideal for handling viscous liquids like oil. The other answer choices are incorrect because: A) maintaining design clearances is still important for proper operation, B) the tooth angles are not an inherent advantage, and C) while stuffing boxes can help reduce leakage, they are not the primary reason these pumps are well-suited for oil.