when talking about viscosity, a 50 weight motor oil is the same as 90 weight gear oil.
im not saying they are the same oils, but viscosity is viscoity when talking about a liquid.
since wikipedia is FOS, here is info from an amsoil site:
http://www.upmpg.com/drivetrain/gearlube/
You should have just posted this part to make it simple.
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 motor oil. However, they are the same viscosity. According to AMSOIL Technical Drivetrain Products Manager Kevin Dinwiddie, the difference is in the additives.
"Motor oil has to combat byproduct chemicals from gasoline or diesel ignition and should contain additives such as detergents and dispersants," said Dinwiddie. "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."
Engine oils and gear oils both have anti-wear additives, and they both must lubricate, cool and protect components, but gear oils are placed under extreme amounts of pressure, creating a propensity for boundary lubrication, a condition in which a full fluid lubricating film is not present between two rubbing surfaces. For example, differentials in cars and trucks have a ring and pinion hypoid gear set. A hypoid gear set can experience boundary lubrication, pressures and sliding action that can wipe most of the lubricant off the gears. To combat this extreme environment, extreme pressure additives are incorporated into the oil. AMSOIL uses an extra treatment of extreme pressure additives in its gear oils in order to reduce wear and extend the gear and bearing life.
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.
