Foaming
The rotating motion of the gear sets also tends to churn the lubricant, resulting in foaming. If a gear lube
foams, the load carrying capacity is significantly reduced because the air suspended within the oil is compressible. For example, when the gear teeth come into contact with each other any trapped air bubbles will compress, therefore reducing the thickness of the separating oil film. In turn, this reduction could lead to direct metal-to-metal contact between gear teeth and result in accelerated wear.
The gear lube must have the ability to dissipate this entrapped air, insuring sufficient lubricating film exists to protect the gears from contact wear. Much like engine oil the chemical compounds, or additives, added to drivetrain stocks either enhance existing properties or impart new ones.