Walk to the oil section of your local auto shop and chances are you'll face a mind-numbing selection of motor oils in a variety of grades from a plethora of manufacturers. If you walk in with no background on the subject of motor oil, it can be difficult to tell each apart or know which is the best motor oil for your beloved auto. Is regular oil better than synthetic motor oil? What do all those numbers on the front of the bottle mean? The good news is it's not as complicated as it seems.
What Motor Oil Numbers Mean
On the label of a quart of motor oil you may see numbers such as 10-40, SAE 30, or 20W-50. These digits correspond to a set of measurable qualities in the motor oil, one being the viscosity index. Viscosity is the oil's resistance to flow. The higher viscosity of an oil (or thicker, if you will), the slower it flows. A lower number refers to thinner, faster-flowing oil.
Single-grade motor oils feature a single number rating and are good for classic or vintage cars. This motor oil doesn't use a polymeric Viscosity Modifier, and instead its viscosity is measured when the oil (non-winter grade) is heated to 210 degrees, which is considered to be the approximate automotive engine operating temperature. Based on its resistance range at this temperature, the oil is then graded as SAE 20, 30, 40, 50, or 60; the higher the viscosity, the higher the SAE grade. A quart of Valvoline Premium Conventional SAE 50W Motor Oil ($5.99 with in-store pickup, a low by $9) is an example of the best single grade motor oil.
Multi-viscosity oils, on the other hand, feature two numbers in their ratings. The left number refers to the oil's behavior in colder conditions, while the right number signifies its behavior in warmer conditions. Multi-viscosity oils also include a 'W' which conveniently stands for "winter." These motor oils were developed to compensate for engine wear that results from temperature extremes.
Today, multi-viscosity oils are used almost exclusively over monograde oils since they function best in changing climates. An SAE 30 motor oil, for example, is consistent with the viscosity of, say, a 5W-30 oil when both are at 210 degrees, but the latter offers better performance in the cold as well. Multi-viscosity oils also contain additives that keep the oil from thinning out as its heated. A Castrol GTX High Mileage 10W-40 Motor Oil 5-Quart Jug ($29.99 with in-store pickup, a low by $1) is a great such motor oil at just $5.88/quart.
Keep in mind that motor oil does differ by brand at times. A 5W-30 oil from one brand may be equivalent to a different brand's 10-40 simply because there is a variance in some of the oil's other properties. Be sure to ask your local mechanic or a friend that knows cars, and buy a trusted, reputable brand.
The Best Synthetic Oil
In the early '70s, AMSOIL brought the first synthetic automotive motor oil to market, and it's still considered to be one of the better oils for diesel motors. As such, a gallon of the Amsoil Synthetic Blend 15W-40 Heavy Duty Motor Oil ($26.40 with $11.84 s&h) is the original synthetic motor oil. Later in the decade, Mobil Oil released its first synthetic motor oil, and by the '90s, other major oil companies had joined in.
While most people will claim that synthetic motor oil blends are better in almost every way, understand that artificial motor oil is also much more expensive. In reality, synthetic motor oil does offer better low-temperature flow and better high-temperature resistance, plus leaves close to zero deposits. Because synthetic oil flows so much better and penetrates better than regular oils, switching to synthetic oil might reveal leaks you didn't even know your car had.
Another one of the best synthetic motor oils, in addition to the aforementioned Amoil, is the Mobil 1 10W-30 Full Synthetic Motor Oil 6-Quart Case ($34.47 via $12 mail-in rebate and free shipping, a low by $18; expires October 31), which will set you back just $7.75/quart.
The Best Motor Oil for Older Cars
As a car ages, gaps between engine parts grow larger, which in turn, cause less oil to reach its critical parts. A lot of experts say that older car owners should switch to a higher-viscosity oil. If your ride is over the 80,000-mile mark, check with your mechanic or someone familiar with your car (or if in doubt, ask the dealership) and determine if you should swap out the current type of oil in your vehicle.
Older vehicles with more miles, more often than not, also have lots of mechanical wear in the engine, which in turn can create internal oil leakage. However, it's not recommended to use full synthetic oil on high mileage vehicles because the oil is often too thin and free flowing. We'd suggest a full-bodied semi-synthetic oil instead. (Almost sounds like we're talking about wine rather than motor oil, eh?) The best semi-synthetic motor oil is a quart of Lucas 10W40 Semi-Synthetic Motor Oil ($7.99 with free shipping, a low by $1).However, full synthetics and semi-synthetics motor oil are more commonly used in high-performance applications when there's an engine with very tight tolerances, high engine temperature, or high compression applications (such as a supercharged or turbocharged motors).
We hope this helped clear some of the confusion as to motor oils and what all the numbers and types mean. By all means, use this as a guide, but check with your dealer or trusted mechanic before making any major switches to your vehicle. That will save you plenty of headache and heartbreak down the road.
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