Interesting info about Gear Oil

[Scott Harrison]

I don't know if this is entirely accurate or not.

Gear Oil

Gear oil viscosity is measured at 150 F vs. 210 F for motor oil. Therefore, 40 W motor oil is the same as 95 W gear oil.

Gear oil is acidic, motor oil is alkiline. Gear oil needs very high wear protection - Extreme Pressure (marked as EP). Therefore, it has a very high sulfer and phospor content. Sulfur and Phosphate reactions start at a lower temperature, and Gear Oil has much more additive than motor oil. This additive is corrosive to copper bearings and bronze synchro rings.

Positraction additives are Friction modifiers - make the base oil much more slippery. They coat the metal surfaces, and prevent the stick/slip mode of operation, preventing shudder, and causes smoother take-up. Friction Modifiers may detract from EP characteristics.

Friction modifiers cause smooth take-up of Limited slip units. For track racing, FM is probably undesirable, and immediate take-up is more important. For Street, FM is usually recommended for more comfortable operation.

Gear oils decompose at lower temperature, usually 250 F.

Gear Lubrication Ratings

   GL-1   No Additives
   GL-2
   GL-3
   GL-4   Suitable for light duty hypoid sets
   GL-5   Has lots of sulfer - Heavy duty hypoids
   GL-6

Hypoid type gear sets have a sliding rather than rolling action, and therefore require much greater wear protection.

GL-5 Should be used in rear differentials.

GL-6 is a heavier weight GL-5. Used for heavy trucks and Tow Vehicles.

Red Line 75 - 90 NS has No Slip, i.e., no Friction Modifiers.
Red Line 75 - 90 has Friction Modifiers.

Gear mesh in Gears literally chops up and cuts apart the long polymer chains of Viscosity Index improvers.

Smell of gear oil is from high sulfur content.

Quaiff Differential (Automatic Torque Biasing Helical LSD) is a worm gear, and needs a very slippery oil.