Motor oils are a complex blend of many ingredients, but they're made up of two primary elements -- base oil and additives. With conventional oil, crude oil is refined extensively until suitable base oil is achieved. Less refined portions of the crude are much thicker and used for different applications, such as roofing tar or road asphalt.
Oil additives perform a number of important functions. For one, they keep oil from breaking down due to high engine temperatures. They also prevent rust and corrosion, improve engine cleanliness, create a film that protect metal parts from wear and improve the oil's flow characteristics.
When you're shopping for motor oil, you'll see each product's performance information listed on the label. For starters, make sure you're looking at the right kind of oil for your vehicle -- S-type oil is for gasoline engines, and C-type oils are used only in diesel engines.
You'll also see viscosity ratings listed on a product's label. High-viscosity oils are thick and flow slowly, while low-viscosity oils are thinner and flow faster.
Oil viscosity changes as an engine warms. As engine temperatures go from cold to very hot, the oil thins, and as a result, its lubricating properties change, too. A 5W-30 oil, for example, has the viscosity of a 5W oil in cold, wintry conditions (the W stands for winter), and a viscosity of 30-weight oil at regular engine operating temperatures. To address this challenge, manufacturers add polymers to oil to make it suitable for a greater range of temperatures.
Synthetic oils handle severe temperatures better than conventional oil. For example, a 0W-30 synthetic oil flows smoothly at -62 degrees Fahrenheit (-52.2 degrees Celsius) and even lower temperatures. In stark contrast, these temperatures basically freeze conventional oil to a standstill. In other words, there's simply no conventional equivalent to this new grade.
This improvement in viscosity and performance didn't happen quickly -- it took many years of research for scientists to make synthetics work well. As scientists developed their new synthetics and blends, new categories of oils emerged.