I'll try once again. The bubbles can be considered foam. Since you don't know the quality of the oil and whether or not it has "foam inhibitors" then you will never know the answer. And bubbles are, or create foam, even though it may not be shown on a dipstick as froth.
Foam Resistance
Foam results from air or other gases becoming entrained in the hydraulic fluid. Air enters a hydraulic system through the reservoir or through air leaks within the system.
A hydraulic fluid under high pressure can contain a large volume of dissolved or dispersed air bubbles. When this fluid is depressurized, the air bubbles expand and produce foam. Because of its compressibility and poor lubricating properties, foam can seriously affect the operation and lubrication of machinery.
Proper foam inhibitors modify the surface tension on air bubbles so they more easily break up.
Here is another quote, it is regarding motor oil but applies to hydraulic oil:
Air is whipped into the oil by rapidly moving engine parts. Air is also trapped in the oil during high-pressure or when the pump sucks air in with the oil. The result is a mass of oily froth called foam. The presence of small amounts of water increases this engine oil problem. The basic cure is an engine design that prevents air from being whipped into the oil and excludes water. Even the best design, however, will not eliminate foaming completely.
A mixure of oil and air is not a good lubricant, cannot support bearing loads, and does not provide good cooling. Because it contains air, oil foam is compressible. This impairs its ability to prevent wear, and the result can be the collapse of hydraulic valve lifters and subsequent poor engine operation.
High-quality motor oils contain foam inhibitor additives that weaken the tiny air bubbles and cause them to collapse almost as soon as they are formed in the oil. These chemical compounds, composed principally of hydrogen and silicon, are called silicones. Only a few parts per million are required to make most oil adequately foam-resistant.
The Motor Oil Guide, The American Petroleum Institute.