Growing up my father owned a service station and I can remember from a very young age that the most popular oil was 10W-40. At the time, I didn’t
even know what those numbers meant. I simply sold the oil that people wanted and left it at that. But as I grew older I came to realize that those numbers meant so much more. Let’s take a brief look at this.
What is viscosity?
In layman’s term, the viscosity is the liquids resistance to flow. It is a lubricant’s most important property. The viscosity must be high enough to maintain a lubricating film, but low enough that the oil can flow around the engine parts under all conditions.
The viscosity index is the measure of how much the oil’s viscosity changes as temperature changes. The higher the viscosity index, the less the viscosity changes with temperature.
Motor oils were given a grade based on the operating temperature of the engine (212°F). For example, an SAE20 operated with a 20 viscosity based on the flow rate at operating temperature. But people discovered that as the temperatures declined during winter, the heavier grades of oil didn’t perform as well as the lower grades, so they would often switch out oils in preparation for winter.
In the 50s, the winter-grade designations were developed to indicate the viscosity at 0°F, essentially a cold-winter start. Because oils developed from crude oil from different parts of the world, reacted different at cold temperatures, a typical SAE20 oil, while operating as a 20 viscosity oil at operating temps, would operate different at a cold start if it was developed from crude recovered from the Gulf of Mexico vs. Pennsylvania crude. Thus the winter designation was given. This lead to the 20W (yes the W stands for winter, not weight as commonly mistaken).
Soon the mutligrade oils, such as the 10W-40 that we all knew and loved back in the day were developed. That number indicates that the oil has a 10 viscosity at cold start, but has a 40 viscosity at operating temps.
Why are we trending lower?
Due to the smaller engines and the rise of the cost of gasoline, the need for improved fuel-economy became paramount. As this occurred, the lower viscosity oils became more important and popular. 10W-30 took the place of 10W-40 in many engines, and by the 90s 5W-30 became the most recommended oil. The lower viscosity has led some to question the ability to protect from wear. This is where quality comes into play. Premium oil usage is key to balancing fuel economy and protection. And this is where AMSOIL comes into play. It’s high quality formulations maintain a strong lubrication between metal parts.
I’ve linked to a document that explains much of what I’ve talked about in this document in a more thorough and informative fashion.