Sucking air and oil at the same time.
(Foamy oil causing Cavitation)

Cavitation occurs when bubbles, or cavities of air, begin to form in the pump.
The internal pump environment should be completely free of air in order to avoid this problem, which often occurs at the inlet of the pump.

Cavitation puts undue stress on pump components, which causes the pump to wear more quickly than normal. It also reduces the useful life of the system's hydraulic fluid.