Interpreting Oil Viscosity Ratings

Few subjects other than equipment-color loyalty bring about more heated discussions than oil brands. Regardless of your brand preference, the Society of Automotive Engineers (SAE) and the American Petroleum Institute (API) have defined the quantifiable characteristics of lubricants that the industry adheres to. Viscosity is one of them. 

This characteristic is defined as a fluid’s resistance to flow. Some think of viscosity as the thickness of oil, but that would only be a partial truth.

Viscosity defines the force required to move a given layer of fluid past another layer at a given speed and standard separation. 

Engine oil is normally a multiviscosity blend such as 15W-40. The W does not stand for weight; it stands for winter. 

Thus, the oil will flow as if it were thin #15 when cold and respond as #40 when hot. Multiviscosity oils were developed years back to provide the best protection to the engine under all temperature extremes.

Viscosity rating doesn’t tell the entire story, though. Many brands of engine oil have the same viscosity. Thus, a convincing argument could be made that there is no difference among brands of oil. That would be incorrect. 

Numerous factors make oil an average vs. a premium lubricant. These factors are based on the crude employed, the refining process, any additives used, plus a host of other factors. For now, let’s focus on viscosity.

Let’s say you were harvesting corn from two locations in the field. The opening of the field produced 200 bushels, while the middle yielded 305 bushels. A true statement would be the ground around the entry to the field yielded 200 bushels, while the middle was 305-bushel land. That is the field’s performance for those two points. That tells you nothing about the yield in the other areas, though. That is what the rating on the oil container is saying.

viscosity rating

The viscosity rating is the oil’s performance at the assigned temperature for cold and hot. The cold temperature test is at 0°F. ; the hot rating is at 212°F. 

For example, 15W-40 oil acts as if it were 15 weight at 0°F. degrees and as 40 weight at 212°F.  You only know its performance at those two temperatures. To get the full story, a visometric index chart is required, and no one publishes that.

Simply put, most inexpensive oils skew greatly in their viscosity above and below the test temperatures, and that is where your engine will experience excessive wear. Using a high-quality oil will offer better performance up and down the temperature range.