Many tests have shown that engine oil pumps are the limiting factor in providing satisfactory high-altitude aircraft oil system performance.
As is pointed out in this paper, it is for this reason that both reciprocating and gas turbine engine manufacturers should design and provide their engines with a pump that will give satisfactory performance at an inlet pressure of 2 in. of Hg absolute with 10% (by volume) entrained air.
Of the various oil systems investigated by the AMC, the closed-circuit system seemed to be most desirable in terms of both performance and installation, for use in high-altitude operational aircraft.