Gear Oil Weight Explained

The measure of the thickness of a liquid is called viscosity. Though there are several different measurements for viscosity, the one standard with gear and other automotive oil is the Society of Automotive Engineers (SAE) weight classification. The larger the SAE grade number the more viscous the oil.

Viscosity is affected by temperature. SAE viscosity ratings are given for the oil as measured at 212 degrees Fahrenheit. As oil gets hotter it becomes less viscous, as it gets cooler it becomes more viscous. Imagine putting honey in the microwave; at high temperature it flows more quickly. For cold weather oil the SAE has a "W" certification for oil that is tested at 32 degrees Fahrenheit. The grade will be indicated with a number followed by the letter "W" to indicate winter viscosity.

The SAE grading system indicates oil viscosity in simple numbers ranging from the thin oil (10) to the very thick oil (140 or above). Heavier weight, or higher grade oil is more viscous and therefore thicker as it flows between the gears. Light grade oil is ideal for small, fast, polished gears under a light load. Heavy grade oil is suited for large, slow, rough gears under a heavy load. Check your manufacture's specifications for the gear grade needed in your equipment.

To account for changing temperatures oil is manufactured through the use of additives so that it can have multiple grades of viscosity, or multiple weights. One weight will be a winter weight and the other will be the hot weight. So you may see gear oil listed as "75W-90" which gives you both the winter and hot weights of the oil. Instead of switching out the oil during the year to compensate for the changing temperatures, most equipment will allow a multi-grade oil to be used year-round (check your manufacturer's specifications to be sure).