One of the most common things we do to our cars is change their oil. But do we really understand what and why we are doing or do we just go through the motions like we always have? For most of us, we usually change our oil every 3000 miles and either use the most economical oil or the best protecting oil. Let me propose that we can do both. First off, we should understand some key concepts.
The most basic concept in motor oil is viscosity but many people don't really have a good understanding of this concept. From an engineering point of view, viscosity is a measure of a fluid's internal resistance to flow, more commonly understood as a fluid's thickness. Viscosity can be measured with a variety of units and a commonly used unit in modern oil spec sheets is the centistoke (cSt).
Motor oil is classified according to its SAE (Society of Automotive Engineers) viscosity rating and most people would be familiar with viscosities such as SAE 10W-30, SAE 5W-30, SAE 30, etc. Standard SAE J300 classifies these viscosities and, without getting into too much explanation, a multigrade oil (like SAE 10W-30) has viscosity characteristics of SAE 10W oil (W oils are tested at low temperatures) as well as that of SAE 30 oil. An oil with a single viscosity number (like SAE 30 or SAE 10W) only meet the one part of the standard. The viscosity specifications of SAE J300 are summarized in SAE Viscosity Grades For Engine Oils.
SAE J300 evolved over the years (first introduced in 1926) and the low temperature portion of the standard was added in 1950. Prior to 1950, motorists selected their oil based on ambient temperature. Multigrade oils were developed to overcome the need for seasonal viscosity changes and motorists could instead plan their vehicle maintenance by mileage only.
The following photo gives a good illustration of the differences between oils of various viscosities. All have been cooled to a low temperature and and they were all tipped at the same time. The 15W-40 has barely started to flow while the 0W-30 is nearly completely finished.