A mixture of air and a petroleum product (e.g., lubricant, fuel oil) which can lower a product's effectiveness and cause sluggish hydraulic operation, air binding of oil pumps, and overflow of tanks or sumps. It can be a result of excessive agitation, improper fluid levels, air leaks, cavitation, or contamination with water or other foreign materials. Foaming can be inhibited with an antifoam agent. The foaming characteristics of a lubricating oil can be determined by blowing air through a sample at a specified temperature and measuring the volume of foam, as described in test method ASTM D 892.