Viscosity

Viscosity (a fluid's resistance to flow) is rated at 0° F (represented by the number preceding the "W" [for Winter]) and at 212° F (represented by the second number in the viscosity designation). So 10W-30 oil has less viscosity when cold and hot than does 20W-50. Motor oil thins as it heats and thickens as it cools. So, with the right additives to help it resist thinning too much, an oil can be rated for one viscosity when cold, another when hot. The more resistant it is to thinning, the higher the second number (10W-40 versus 10W-30, for example) and that's good. Within reason, thicker oil generally seals better and maintains a better film of lubrication between moving parts.

At the low-temperature end, oil has to be resistant to thickening so that it flows more easily to all the moving parts in your engine. Also, if the oil is too thick the engine requires more energy to turn the crankshaft, which is partly submerged in a bath of oil. Excessive thickness can make it harder to start the engine, which reduces fuel economy. A 5W oil is typically what's recommended for winter use. However, synthetic oils can be formulated to flow even more easily when cold, so they are able to pass tests that meet the 0W rating.

Once the engine is running, the oil heats up. The second number in the viscosity rating--the "40" in 10W-40, for example--tells you that the oil will stay thicker at high temperatures than one with a lower second number--the "30" in 10W-30, for example. What's really important is that you use the oil viscosity your car's owner's manual recommends.

Viscosity Index

Resistance to thinning with increasing temperature is called viscosity index. And although a higher second number is good, the oil also has to be robust. That is, it must be able to last for thousands of miles until the next oil change. For example, oil tends to lose viscosity from shear, the sliding motion between close-fitted metal surfaces of moving parts such as bearings. So resistance to viscosity loss (shear stability) is necessary to enable the oil to maintain the lubricating film between those parts.

Unlike antifreeze, 95 percent of which is made up of one base chemical (typically ethylene glycol), petroleum-type engine oil contains a mixture of several different types of base oil, some more expensive than others. Oil companies typically pick from a selection of five groups, each of which is produced in a different way and in different viscosities. The more expensive groups are more highly processed, in some cases with methods that produce a lubricant that can be classified as a synthetic. The so-called full synthetics contain chemicals that may be derived from petroleum but they're altered so much that they're not considered natural oil anymore. Our custom blend contained 10 percent polyalphaolefins (PAO), the type of chemical that's often the primary ingredient in a full synthetic.

The base oil package in any oil makes up anywhere from 70 to 95 percent of the mix, the rest comprised of additives. Does that mean an oil with just 70 percent base oils is better than one with 95 percent. No, because some of the base oils have natural characteristics or ones that derive from their processing, which reduces or eliminates the need for additives. And although some additives make important contributions to lubrication, by themselves don't necessarily have great lubricity.

The ingredients in an additive package differ in cost, as we said, but price is just one factor. Some work better in certain combinations of base oils, and some of the less-expensive base oils are a good choice for a blend because of the way they perform with popular additives. Bottom line: every motor oil has a recipe. Refiners come up with a list of objectives based on the needs of their customers (the carmakers, for example) and formulate oil to meet those goals as best they can.

Now, keeping an oil from thinning as it gets hot while it takes a beating from engine operation is one thing. But it's also important to keep oil from getting too thick. Using premium base oils for low volatility--to prevent evaporation--is one approach. Evaporation of the base oil package not only increases oil consumption, it results in thicker oil (which decreases fuel economy).

Oil Additives

Use of additives is another approach to improving and maintaining oil performance. High engine temperatures combine with moisture, combustion byproducts (including unburned gasoline), rust, corrosion, engine wear particles and oxygen to produce sludge and varnish. The additives not only assist oil in maintaining good lubrication, they also help minimize sludge and varnish, and any damage from their formation. Here are the categories of key additive ingredients and why they're important:

• Viscosity-index improvers: Reduce the oil's tendency to thin with increasing temperature.
• Detergents: Unlike the household type, they don't scrub engine surfaces. They do remove some deposits, primarily solids. But their main purpose is to keep the surfaces clean by inhibiting the formation of high-temperature deposits, rust and corrosion.
• Dispersants: Disperse solid particles, keeping them in solution, so they don't come together to form sludge, varnish and acids. Some additives work both as detergents and dispersants.
• Antiwear agents: There are times when the lubricating film breaks down, so the antiwear agents have to protect the metal surfaces. A zinc and phosphorus compound called ZDDP is a long-used favorite, along with other phosphorus (and sulphur) compounds. If you musts know, ZDDP stand for zinc diakyl dithiophosphate.
• Friction modifiers: These aren't the same as antiwear agents. They reduce engine friction and, so, can improve fuel economy. Graphite, molybdenum and other compounds are used.
• Pour-point depressants: Just because the 0° F viscosity rating is low doesn't mean the oil will flow readily at low temperatures. Oil contains wax particles that can congeal and reduce flow, so these additives are used to prevent it.
• Antioxidants: With engine temperatures being pushed up for better emissions control, the antioxidants are needed to prevent oxidation (and, therefore, thickening) of oil. Some of the additives that perform other functions also serve this purpose, such as the antiwear agents.
• Foam inhibitors: The crankshaft whipping through the oil in the pan causes foaming. Oil foam is not as effective a lubricant as a full-liquid stream, so the inhibitors are used to cause the foam bubbles to collapse.
• Rust/corrosion inhibitors: Protect metal parts from acids and moisture.

More Is Not Better

You can't necessarily improve an oil by putting in more additives. In fact, you can make things worse. For example, sulphur compounds have antiwear, antioxidation characteristics, but they can reduce fuel economy and affect catalytic converter operation. Too much of a particular detergent could affect the antiwear balance. Too much of a specific dispersant could affect catalyst performance and reduce fuel economy. Antiwear and friction-reducing additives also may have ingredients (such as sulphur) that could affect catalyst performance.

There's a lot of pressure on the oil industry to reduce sulphur content in oil as well as gasoline. But the industry's resistance is understandable when you consider the delicate balancing act it must perform with each revolution of your car's engine.

© Popular Mechanics Magazine, August 2002