5 Engine Oil Myths

When you buy engine oil, it's important to know the oil's viscosity, a property that corresponds roughly to its thickness. The less viscous the oil, the more smoothly it moves through your engine and lubricates the moving parts. The best engine oils have a viscosity that is neither so high (thick) that it will barely flow or so low (thin) that it will slip through your engine like water.

There are two ways in which oil viscosity is measured: single grade and multi-grade. SAE 30 is a typical single-grade rating. That means that an organization called the Society of Automotive Engineers (SAE) ran the oil through a standardized tube-like device and timed how long it took, in seconds, to flow from one end to the other. The viscosity rating is the number of seconds rounded to the nearest multiple of ten. Thus, SAE 30 oil takes approximately 30 seconds to flow through the tube. This single viscosity rating is sometimes called the oil's "weight."

Unfortunately, oil changes its viscosity with temperature and the single viscosity rating only represents the flow of oil when it's warm. What if you need to start your car on a cold winter morning? The oil will flow more slowly, so the cold viscosity rating is important too. A multi-grade rating gives you both the hot and cold viscosities. For 10W-30 oil, the 30 is the same as the SAE 30 viscosity rating for warm oil, but the 10W is the viscosity rating for cold oil, according to a standardized rating system developed by the SAE for winter oil use.

And that's what the "W" stands for: "winter."

If you're conscientious about keeping your car in good running order, you probably worry from time to time that your oil has gotten dirty and is causing sludge to build up in your engine. So you pull the dipstick out and check the color of the oil at the tip. Chances are, it's starting to turn dark, no longer the light amber color that you saw on the stick when your oil was fresh. So now it's too dirty to use, right? It's depositing sludge in your engine and needs to be changed.

Wrong. In fact, just the opposite is true. If you're using a detergent engine oil (and most modern engine oils have detergent additives), the oil is working just the way it's supposed to, dispersing the tiny particles that can result in engine sludge and holding them in suspension in the oil itself so that they can't build up. That's why the oil appears darker, but this in no way impedes the oil from performing its normal functions of lubricating and protecting the metal surfaces inside the engine. Of course, the oil is limited in how many of these suspended particles it can contain and will eventually need to be changed when it becomes saturated, but use the oil change interval recommended by your car's manufacturer to decide when to change the oil, not the color of the oil on the stick.

Once upon a time, almost every auto manufacturer recommended that the oil in your engine be changed every 3,000 miles (4,828 kilometers). Use oil past that interval and the engine would begin to fill with sludge, which would not only degrade performance but leave the moving parts at risk for damage.

That's no longer true. Modern detergent oils, improved oil viscosities and better auto engineering in general now allow cars to go about 7,500 miles (12,070 kilometers) between oil changes. Yet you'll still hear the 3,000-mile (4,828-kilometer) figure quoted widely -- especially by people trying to sell you oil. No less an authority than Consumer Reports has debunked this myth, stating that unless you drive your car under unusually difficult conditions, and especially if you always drive it in stop-and-go traffic, going 7,500 miles (12,070 kilometers) between oil changes shouldn't harm your engine in any way.

This is true -- except that these "additives" have already been added before you buy the oil. Any reputable brand of motor oil will come with additives that improve its viscosity index -- the range of temperatures under which it flows properly through the engine -- and that give it detergent properties that keep your engine free of sludge. Most will also include rust retardants to prevent corrosion and chemicals to protect metallic surfaces.

With all these additives already in the oil, putting in more may actually dilute what's already there and lessen the oil's effectiveness. Check your car's manual to see if it has any special additive needs, but this is unlikely in anything except some of the most exotic high-performance engines.

Back in the 1970s, when synthetic engine oils (those based not on petroleum but on chemical base stocks such as polyalphaolefins) first became popular, they didn't always play well with the seals and gaskets in the car's engine. They could cause the seals to shrink in ways that petroleum-based oils did not, resulting in those messy oil leaks that would mysteriously appear in your car's parking space. Some people still fear that synthetic oil will cause leaks and so they continue to use petroleum-based oils instead.

These fears are largely unfounded. Oil manufacturers long ago learned to reformulate synthetic oil so that seal shrinkage doesn't occur. Still, there's a way in which synthetic oil can cause a leak, at least when you use it in an older car that's been operating for years on a petroleum-based oil. The synthetic oil can clean oil sludge off the seals that may actually have been blocking off tiny cracks in the seals, revealing leaks that have been there all along. This probably won't be a problem on newer cars, but if you're still driving a car that's more than, say, 15 years old, you might not want to make a sudden decision to switch to a synthetic oil.