Back in the days of R11 I used a small, one-piece pump that I believed was intended for "pool duty". It had a direct drive rubber impeller. It worked fine until R123, and its lower pressure, appeared on the scene.The vacuum in the oil sump was to great for that pump to overcome. Then I bought the smallest coupled gear pump (and motor to drive it) that I could find in Grainger's catalog. In 25 years those were the only pumps I used for that purpose. Although that little pool pump got to where it would sometimes spark pretty good while running! lol, ahhh, good times.