The viscosity of a fluid is the measure of a fluid’s resistance to deformation and movement. The higher a fluid viscosity, the slower the fluid flows and the lighter the viscosity, the easier the fluid flows. Measured viscosity of a lubricant changes with temperature. As the temperature of the engine oil or gear oil increases, the viscosity decreases, and vice versa.
The viscosity grade of the lubricant is the number listed on the label of the oil bottle. For motor oil and gear oil, the viscosity grade is listed in the SAE grade scale, and will most commonly appear as a number with a ‘W’ followed by another number. For example, 5W-20 and 10W-30 are common SAE viscosity grades for engine oil, while 75W-90 and 85W-140 are common viscosity grades for gear oils. The viscosity grade of the oil is a representation of the measured viscosity at certain measuring temperatures. The last number of the SAE viscosity grade, which is most commonly between 20 and 50, is determined at 100°C (212°F). This is the most important part of the SAE viscosity grade as it provides an indication of the viscosity of the oil at “engine operating temperature”. All lubricants with the same last number are referred to as multi-viscosity grades of that number. For example, 0W-30, 5W-30, and 10W-30 are all multi-viscosity 30 weight engine oils and 75W-90 and 80W-90 are all multi-viscosity 90 weight gear oils. The number with the ‘W’ is called the Winter Grade and the ‘W’ stands for ‘winter’. This portion of the viscosity grade is determined at a very cold temperature between -10°C and -40°C for motor oils and between -12°C and -55°C for gear oils. The smaller the winter grade
While the measured viscosity of a lubricant changes with temperature, the viscosity grade does not. A SAE 5W-30 is always a SAE 5W-30 oil no matter what the temperature of the oil. The viscosity grade of the lubricant does not change unless the oil becomes damaged or contaminated by another substance. The viscosity of lubricants with the same last number in the SAE grade will exhibit similar measured viscosity at typical operating temperature, but the lubricant with the lower winter grade (5W vs 20W, or 75W vs. 85W) will flow better at colder temperatures. Regardless of the numbers listed on the bottle, all automotive gear oils and engine oils become more viscous (‘thicken’) as they cool and become less viscous (‘thin’) as they get hotter. A SAE 10W-30 engine oil does not go from a ’10 weight’ to a ’30 weight’ as it warms up. The oil is always a SAE 10W-30 regardless of temperature.