Here's a good article on Motor and Gear oil...
Gear Oil and Motor Oil Are Not the Same
Gear oil differs from motor oil. Most people assume that SAE 90 gear oil is much thicker than SAE 40 or 50 grade motor oil. However, they are the same viscosity. The difference is they are calculated by different classifications, SAE gear lube and SAE engine oil. Another main difference is the additives used to produce them.
Motor oil has to combat byproduct chemicals from gasoline or diesel ignition and should contain additives such as detergents and dispersants. Since an internal combustion engine has an oil pump and lubricates the bearings with a hydrodynamic film, the need for extreme pressure additives such as those used in gear oils does not exist in engines.
Additional Differences
Because many of the components found in the drivetrain consist of ferrous material, the lubricant is required to prevent rust and possible corrosion to other materials. Rust and corrosion problems are not nearly as prevalent in engines.
The many small and intricate components that make up gear sets found in the drivetrain can be quite noisy and may be subjected to shock loading. The viscosity and extreme pressure formulation of gear oil quiets gears and dissipates shock loading.
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 oil must have the ability to dissipate this entrapped air, insuring a sufficient lubricating film exists to protect the gears from contact wear.
Typical Drivetrain Fluid Additives
Much like engine oil, the chemical compounds, or additives, added to drivetrain base stocks either enhance existing properties or impart new ones. Some of the additives that may be found in a drivetrain fluid include the following:
• Extreme pressure and/or antiwear agents - These additives are used to minimize component wear in boundary lubrication situations.
• Pour point depressants - This type of additive is used to improve low temperature performance.
• Rust and corrosion inhibitors - These are used to protect internal components.
• Oxidation inhibitors - These additives are used to reduce the deteriorating effects of heat on the lubricant, increasing the lubricant’s service life.
• Viscosity index improvers - These allow a lubricant to operate over a broader temperature range.
• Anti-foam agents - These are used to suppress the foaming tendency and dissipate entrapped air.
• Friction modifiers - The required degree of friction reduction can vary significantly between differing pieces of equipment in drivetrain applications. In some cases, friction modifiers may be required to obtain the desired results.
