Clean Up The Myths

Gear Oil

Q—What is the primary function of gear oil?

Gear oil is a lubricant specifically designed for transmissions, transfer cases and differentials in cars and trucks. Gear oil has two primary functions: to remove heat from the gears, and to lubricate the gears.

Most gear oils have a very small amount of limited-slip additive for well-broken-in differentials, while Red Line contains additional frictional modifier suitable for newer and aggressive limited-slip units. It works with the friction plates in the couplings that connect the axle shaft to the differential and transfer the power to the wheels. 

The friction plates contain different types of frictional material, each one having slightly different frictional and wear characteristics. The limited-slip additive must work within the materials’ capabilities. This becomes more important in diesel applications because the demands on the rear axles increases with the high-power output and increase in load. Having the proper gear oil and limited-slip additive can eliminate noise, vibration and grabby gears.

Please note that today there are two primary standards for gear oil. The first is GL-4 for manual transmissions and some transfer cases for copper-containing metals. The second is GL-5 which is designed for some transmissions and transfer cases, but is usually found in a rear differential. It contains a more chemically reactive anti-wear which could harm yellow metals or copper-containing metals if not specially formulated for transmission use.

Q—How is this function affected in cold climates? In warm climates?

Using a synthetic multi-weight gear oil like Red Line helps gears function better in both warm and cold weather. More importantly, it is what you are doing with your vehicle and the power.

Heavier gear oils are used for heavy loads, shock loads, low speeds, and rough surfaces because they provide a thicker oil film.

Lighter weight gear oils are used in high speed and lower load applications for better cooling.

Q—How often should gear oil be changed?

For vehicles under warranty, most recommend checking your transmission, transfer case and front/rear differential at 15,000 miles and full-service at 30,000 miles. Once past warranty 40-50,000 miles would be a suitable maintenance interval.

Q—What causes gear oil to break down?

The four most common problems are oxidation, thermal breakdown, contamination and additive depletion.

Oxidation is when oil molecules react to oxygen, causing gear oil additives to deplete and breakdown.

Thermal Breakdown is designed to dissipate heat so it will be exposed to higher temperatures at point of contact.

Contamination is where dirt, water, air, etc. all contribute to the oil’s degradation.

Additive Depletion is when the additive package is only designed to last so long in operation. Heat, contamination, etc., will deplete the additives. The oil and the additives are sacrificial to protect the application.

Q—Are there any signs that tell you your gear oil needs to be replaced?

The four things you need to be aware of are: jerking/slipping, foul odor, gear whining and your service interval.

Jerking/slipping indicates oil is low or past its useful life (this could be fixed by adding or changing the gear oil, but gears could also be damaged and need to be replaced). A foul odor beyond normal gear oil sulfur smell could be from overheating the differential due to lack of or poor lubrication.