Gear Oil EP Additives Explained

This blog article will briefly discuss EP additives used in gear oils, why they are used, and what EP additives are typically used in automotive gear oil lubricants. We will also highlight how they play an important role in protecting the gears in differentials, some transfer cases, and some transmissions. We also offer some suggestions on how to prepare the gear oils before adding them to your differential, transfer case, or manual transmission. 

Gear Oil Extreme Pressure Additives Explained

Many automotive gear oil lubricants use EP additives. This is an abbreviation for “Extreme Pressure” additives. The extreme pressure additives used in gear oils are typically organic or inorganic compounds known as organic sulfur, chlorine compounds (Sulpher-phosphorus), and boron compounds. These EP additives mentioned are almost always associated with an API GL-5 gear oil specification and about 50 percent fewer EP additives in GL-4 gear oil formulations. EP additives are added to many gear oils for the performance benefits of decreasing wear on highly loaded gears used in select differentials, transfer cases, and transmission applications.

Extreme pressure additives together with the gear oil lubricant help form a chemically bonded lubricating film. When the vehicle is operating, the internal gears and components are moving and coming in contact with each other creating friction. This friction increases the temperature in the gear oil triggering a chemical reaction with the EP additives in the gear lube. As the temperature increases as a result of the gears moving over each other, the polar molecules of the EP additives in the gear oil will begin to bond to the gears and various metal components inside a differential forming an extreme-pressure protective film layer on the metal face surfaces. This minimizes any metal-on-metal contact that could cause surface degradation or abrasion to the gears. EP additives in gear oil are extremely important when vehicles are subjected to towing heavy loads. Especially during takeoff starting from dead stop conditions when the extreme pressure on the differential gears and internal components is the greatest. You could say EP additive compounds in the gear oil act like the analogy of a force field which is preventing the metal-on-metal contact of the differential’s gears and internal components.

EP additives are polar molecule compounds. Polar molecules are a molecule atom that has a slightly positive charge, while the other end has a slightly negative charge. EP additive polar molecule compounds, depending on their type and size can dissolve in the gear oil. However, they can also remain as suspended solids due to the gear base oil having some nonpolar characteristics. Hence, the EP additive solids can occasionally be visible when dispensing the fluid into the differential. A term in the industry has labeled this suspended solid condition as “fall out”. This condition does not mean that the gear oil is defective or contaminated. It’s just the chemistry dynamics of extreme pressure additives and the base gear oil formulation mixing. As the gear oil mixes in the differential during vehicle operation and has its intended reactions, the lubricant and additives become soluble and mixed. Remember, extreme pressure additives help prevent damage to the gears and allow the gear oil to perform well over a wide range of temperatures, loads, and speeds. This includes any extreme conditions the vehicle may be subjected to.

Gear Oil Change Tech Tip

A simple tech tip we recommend in helping minimize EP additive separation is shaking the bottle for about 20-30 seconds before adding the new fluid to the differential, transfer case, or manual transmission.

Why Do EP Additive Gear Oils Smell?

The unpleasant smell can commonly identify gear oils with EP additives. This unpopular reeking smell is the result of the EP additive compounds added to these specific gear oil types (GL-5). Whereas gear oils without EP additives or low EP gear oils (GL-4 or GL-4/GL-5 spec), automatic transmission fluids (ATF), some manual transmission fluids (MTF), and motor oils are not as stinky as EP gear oils since low levels of EP additives, or non-sulfur additives are used.

Why Choose Blauparts for All Your Vehicle Lubrication Needs?

BLAU® differential fluid change kits use high-quality, fully synthetic* gear oils and transfer case fluids. Our fluids are sourced from lubricant partners that employ sustainable efforts to improve the environment using a higher level of quality. In turn, this can support the conservation of the environment through less driveline fluid changes and waste, optimize vehicle efficiency and performance, improve and maintain fuel economy, reduce exhaust/carbon emissions, and improve performance through reduced coefficient of friction and less driveline rolling resistance.

*For most kits, check the product description.

Technical Advantages:

  • BLAU® differential fluid change kits conveniently include the correct fluids, gear oil, fluid specifications, and quantities for your vehicle's transfer case and differential fluid change needs
  • All kits use differential and transfer case fluids that meet or exceed your vehicle manufacturer's specific fluid specification requirements
  • Advanced gear oil and transfer case fluid formulation technologies engineered with improved lubricating properties
  • High-quality fully synthetic gear oils and transfer case fluid formulations allow for extended service life, conserving Earth's resources
  • Better extreme pressure (EP) properties, shear stability, lubricating film strength, and oxidation resistance even when exposed to high temperature and high load pressure
  • Optimize driveline efficiency and fuel-saving characteristics with higher-quality gear oils and transfer case fluids
  • Excellent cooling capacity for better driveline performance and less wear 
  • Low foaming properties and resistance to internal differential and transfer case fluid cavitation
  • Great cold-temperature lubricating properties and flow for added driveline protection and efficiency during the cold driving phase