Is an SAE 10W-30 motor oil thinner than an SAE 10W-40? Thicker than an SAE 5W-30? Well, yes, and no. What do those numbers mean? And which is better for your car or truck? It depends.

First, the numbers are ratings of viscosity -- the rate of flow of a fluid -- based on SAE (Society of Automotive Engineers) specified testing. The higher the number, the thicker the fluid, the more resistance to  the flow. And because liquids thin out as their temperatures rise, thicker is better to maintain effective lubrication when they heat up inside a running engine.

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Some simple engine oils have a single rating (Mono-grade), say SAE 30, which is thinner than an SAE 40. So the latter will retain viscosity and lubricate better at higher temperatures. In a high-revving, hot-running racing engine, for example, you don't want your oil flowing like water. But thicker, slower-flowing oil is not good when an engine is cold. Following a cold start, it takes longer to find its way throughout the engine, leaving key parts insufficiently lubricated and exposed to wear for the first few seconds. That is why today's "multi-viscosity" oils are engineered to protect an engine throughout its full range of operating temperatures, from extreme winter cold starts to long, hot, high-speed runs on the hottest summer days.

And that is why they have two ratings. The first number, followed by a "W" for "winter," is a measure of an oil's viscosity at sub-zero temperatures, and lower viscosity gives better flow when cold. "I did some tests in upper Canada," relates Quaker State technology director Jeff Hsu, "and it took three seconds for the oil to get pumped up to the top. So that can amount to six seconds each day you go to work and come back in very cold weather. Multiply that by five times a week over a 5,000-mile drain interval, and that adds up to quite a few minutes."

The second number reflects the oil's viscosity at high temperatures, based on tests at 100C (212F) -- considered "normal" operating temperature -- and 150C (302F). And this is where higher is better to maintain effective lubrication at sustained elevated temperatures.

How do oil companies formulate multi-viscosity lubricants for all-temperature protection? "We add viscosity modifiers (VMs) to conventional oils to artificially provide that range," Hsu explains. "These are little polymer coils, engineered from rubber or plastic resins, that curl up when cold, then expand with temperature. In an SAE 5W-30 oil, for example, they are compressed to give the 5W at low temperatures, then expand when heated up to give 30-weight protection."

The downside is that those polymer coils tend to sheer out -- get chopped up into smaller pieces -- by sliding surfaces and meshing gears in the engine. That reduces their effectiveness over time, and eventually they can blow past the piston rings and leave deposits.

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Synthetic oils have the advantage of natural "viscosity index improvers" (VIIs) so don't require many (if any) polymer VMs to achieve sufficient temperature range. "With a synthetic oil that is more pure with more natural VIIs," Hsu says, "you can optimize it by reducing the number of those coils so it will stay true to its viscosity longer and reduce oxidation quite a bit."

Another advantage of a good synthetic is that it can be engineered to deliver the ultra-low viscosity ratings that automakers are increasingly specifying to achieve better fuel economy. "A lot of them are now calling for 5W-20, 0W-20, even 0W-16," he relates. "With that trend toward thinner and thinner oils, you still have to be able to keep the engine protected, and only a synthetic oil is capable of optimizing these low viscosities."

One more excellent reason to always refill with oil of the type and viscosity range specified by your vehicle's manufacturer.


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