HI ALL!
Well the basic issue for the problem has not been addressed and thus I feel it is partinent to mention what I know about the problem, and its possible solusion!
The air entrapment is due to the psycal activity of filling the oil at high pressure and may be at high temperature also into the barrel. Ususally a good quality oil should have a reasonable Air-Release Value for it so that the entraped air bubbles picked-up during the oils filling / churning operations should get released from the body of the oil and thus very little is left as traped air inside the oil.
However, if this aspect is not addressed during the formulation / QC of the finished product, then to pass the Foaming Characteristics of the oil, defoament is added and that may jeoperdise the Air- Release Value for the oil. Thus there has to be a balence of both the properties to have a good overall quality of the oil, whether it is a Industrial Gear Oil or a Hydraulic Oil!