The problems come when the oil is thick, clearance in the pump may be too much, or the lines on the intake side are larger external, which provide a lot of dead air before oil is pulled into the pump gears . With a. Wide clearance like i found on my BRAND NEW milodon highvolume pump, even a high speed large drill had a very hard time pulling a prime. Once oul drains off the pump rotors too much leakage by the extra clearance caused my deal much grief. Two hours of hand sanding the aluminum pump body to reduce clearance may help. I sanded it down from .004 to .0017 clearance, which should help a great deal. I will reporrt back.