A quick bit of information for those that don't know, almost all pumps designed specifically to move oil of some sort for whatever purpose are positive displacement pumps. Any pump that is designed to produce more than 20 or 30 PSI will be some sort of positive displacement pump. While a lot of modern automotive electric gasoline fuel pumps are positive displacement, they are still unfit for oil transfer due to the high rpm's that they operate at. A positive displacement pump differs from other pumps by the mechanism that actually moves the fluid. Positive displacement pumps have a chamber that continuously decreases in volume forcing the fluid through the outlet. In a positive displacement pump if the fluid cannot move out of the pump something must give. Due to this design it enables them to also be capable of drawing fluid into itself to a certain degree depending on the efficiency of the pump. A good pump with low tolerances will draw a pretty decent vacuum but to do so they must be primed first. Since the fluid has no choice but to leave the pump it will create a vacuum once that fluid has been displaced that will draw more fluid into the pump. If they are not primed, air is quite thin and can easily slip past the tolerances inside the pump and it will sit idling away doing nothing. Same can be said with a worn pump that has loose tolerances. Getting one to self prime while trying to pull a 3 or 4 ft head will be nearly impossible, and even a worn pump that has been primed may not pull more than a few ft of head.
For those that already know, don't think I'm trying to be patronizing, just like to put the info out for those reading that don't know.
