Gear oil is a lubricant made specifically for transmissions, transfer cases, and differentials in automobiles, trucks, and other machinery. It is of a high viscosity and usually contains oregano sulfur compounds. Some modern automatic trans axles do not use heavy oil at all but lubricate with the lower viscosity hydraulic fluid, which is available at pressure within the automatic transmission. Gear oils account for about 20% of the lubricant market.

  • EP additives which contain phosphorus/sulfur compounds are corrosive to yellow metals such as the copper and/or brass used in bushings and synchronizes.
  • GL-1 class of gear oils does not contain any EP additives and thus finds use in applications which contain parts made of yellow metals.
  • GL-5 is not necessarily backward-compatible in synchronous-mesh transmissions which are designed for a GL-4 oil.
  • GL-5 has a lower coefficient of friction due to the higher concentration of EP additives over GL-4, and thus synchronous can not engage as effectively.

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