Not So Unconventional

Synthetic engine oils offer numerous advantages that could save you money
By John Baxter

If you’re thinking about making the switch from conventional motor oil to a synthetic but are apprehensive, you may be pleasantly surprised at the overall results.

To understand the differences between synthetic and conventional oils, we first need to understand viscosity and a close relative, the viscosity index.

Viscosity is simply the thickness of a liquid. Engines need oils thick enough to stick to engine parts and form a film that is strong enough to keep the parts from touching one another. Engines are lubricated by oil stored in the oil sump on the bottom of the engine. The parts are almost dry when the engine’s been sitting. While oil with a high viscosity helps protect engine parts once it gets to them, there is a limit to how thick the oil can be and still circulate through the engine quickly enough. So what your engine really needs is oil that is just thick enough to do the job.

All oils get thicker as they get colder. The particular thickening characteristic of an oil is its VI, or viscosity index. “Synthetic oils, such as Mobil Delvac Synthetic lubricants, can usually achieve higher VI than comparable mineral oil-based lubricants,” says Yeong Kwan, commercial vehicle products offer advisor for ExxonMobil Lubricants and Petroleum Specialties Co. Doctored up “VI improvers,” as all approved engine oils are, they can be thick enough to lube the engine at more than 200 degrees Fahrenheit, while pumping effectively in extreme cold.

Viscosity is expressed with two numbers. Most truckers use 15W-40. The 15W refers to the oil’s thickness at cold temperatures, while the 40 refers to its thickness at normal operating temperatures near 200 F. The synthetic oils sold for use in truck diesels normally have a remarkable viscosity rating of 5W-40.

The problems synthetics solve
Providing oil with the right thickness would be easy if engines always ran at the same temperature, but they don’t. When an engine sits overnight it will reach outside temperature and may need to start in extreme cold. As improved injection systems with electronic controls have become the norm, engines can easily be started in very cold temperatures. So, ideally, if you live in a cold climate, your oil should be able to pump quickly through the engine even at zero.

While the large sump and the massive oil cooler used on truck diesels keep the bulk of the oil slightly higher than 200 F under cruise conditions, the oil that lubricates the pistons and rings normally gets even hotter – nearer 300 F. Sump oil subjected to high temperatures will gradually break down, or oxidize. This often thickens the oil to the point where it will no longer circulate and may even thin it out. Either way, the engine suffers.

“The challenge is to formulate a lubricant that will maintain the desired viscosity during start-up while maintaining the appropriate lubricating capability at high temperatures as well,” Kwan says. “Thus, high viscosity index (VI) is a key parameter.”

And because synthetics also handle extremely high temperatures very well, Dan Arcy, OEM Technical Manager for Shell Lubricants Global Solutions, says, “Synthetics stand out in extreme

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