by Robert Sanborn on Tue, Jul 3, 2007, who was a member of the STLE Society of Tribologists and Lubrication Engineers for 15 years.
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.