A lubricant’s viscosity is arguably its most important property – its resistance to flow and shear. Viscosity can be influenced by several factors, from contamination to temperature. Multi-viscosity oils have been developed to cater to a wider range of grades and temperatures.
Ford explains that the Society of Automotive Engineers (SAE) developed numerical coding system to grade engine oils according to their viscosity. As a general rule, the lower the number, the lower the viscosity. While single-viscosity or mono-grade engine oil works well in a narrow temperature band, swings in temperature may require a multi-viscosity oil.
“Multi-viscosity oils flow differently at different temperatures,” explains Callum Ford, National Marketing Manager at Lubrication Engineers (LE) South Africa. “The change in an oil’s viscosity due to a temperature change is called the viscosity index (VI). Understanding VI is important to understand whether the lubricant in question meets the requirements of your equipment based on the operating temperature range. For example, if you’re operating trucks that transport ore on mines in Siberia, the temperatures they need to function at inside the mine and outside in the cold can differ substantially. If you’re using a mono-grade engine oil, the truck engine is likely to seize once it exits the mine.”
As an example of a multi-viscosity oil Ford cites an SAE 80W-90 oil, which conforms to the SAE 80W requirement at low temperatures and is in the range provided for SAE 90 at high temperatures. “It gets quite complicated, but essentially dynamic viscosity tends to be measured in one-hundredths of a poise and symbolized by cP,” says Ford. “Each low temperature gear lubricant (SAE grade followed by a W) must not exceed a maximum viscosity of 150,000 cP at the temperature indicated. A 150,000 cP fluid appears like a semifluid grease, which will barely pour.”
When it comes to selecting axle or transmission lubricants, one needs to consider both the lowest and highest service temperatures. The SAE 70W, 75W, 80W and 85W oils are for low temperatures and the SAE 80, 85, 90, 140 and 250 are for the high-heat applications.
Beyond considering temperature, Ford says it’s also important to take into account other operating conditions. “The American Petroleum Institute’s (API) “GL-5” designates the type of service characteristics of gears, particularly hypoid in passenger cars and other automotive equipment operated under high speed/shock load, high speed/low torque and low speed/high torque conditions,” he explains. “This is considered an extreme pressure or EP designation. Thus, we have an SAE J306, MIL-L-2105, API GL-5 EP (extreme pressure) gear oil specification.”
The requirements of many equipment manufacturers have exceeded the API GL-5 specification. As a result, SAE and ASTM have proposed updated GL categories for present and future needs. This prompted the development of the API MT-1 category.
API MT-1 designates the type of service characteristics of non-synchronized manual transmissions used in buses and heavy-duty trucks. Lubricants that meet the API MT-1 requirements provide protection against the combination of thermal degradation, component wear and oil seal deterioration not provided by lubricants of API GL-1 through GL-5 quality.
“We offer a variety of gear oils to meet every requirement. Our MIL-L-2105, API GL-5, and API MT-1 product is the LE 703-704 Monolec Gear Lubricant. LE oils that meet all performance criteria for MIL-L-2105, except for the specified viscosities, are 1602-1609 Duolec Vari-Purpose Gear Lubricants and 9920 SYNOLEC Gear Lubricant. MIL-L-2105 oils are recommended by Mack Truck, Rockwell International (now Meritor), Eaton Axle, Clark Axle, Navistar, General Motors, Ford, Chrysler and Caterpillar for many applications,” says Ford.
If confused about the best gear oil for your particular application, Ford suggests consulting with a lubrication technician, who will be able to advise you on the best product for your needs.
Multi-viscosity oils have been developed to cater to a wider range of grades and temperatures.