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How to Choose the Best Motor Oil for Your Car

What's the difference between regular and synthetic, and what do all those numbers on the label mean? We cut through the confusion.
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best motor oil

If you're looking to score a motor oil deal, chances are you'll face a mind-numbing selection of options in a variety of grades from a plethora of manufacturers.

If you walk in with no background on the subject, it can be difficult to tell each apart or know which is the best motor oil for your beloved auto. Is regular better than synthetic? 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 the "thicker" it is, if you will), the slower it flows. A lower number refers to thinner, faster-flowing oil.

Single-Grade Oils
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.

SEE ALSO: 8 Simple Auto Repairs That Save Big Bucks in the Long Haul

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 ($6.90 with $5 s&h, a low by $1) is an example of the best single grade motor oil.

Multi-Viscosity Oils
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 ($17.97 with free shipping via prime) is a great such motor oil.

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.

Synthetic motor oil blends are much more expensive, but switching to synthetic might reveal leaks you didn't even know your car had.

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 Amsoil, is the Mobil 1 10W-30 Full Synthetic Motor Oil 5-Quart Jug ($25.47 with in-store pickup).

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 ($11.92 with free shipping via Prime).

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.

What do you look for when choosing a motor oil? Let us know in the comments below!


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Staff Writer

Dan Leadbetter is a Staff and Features Writer for DealNews. He enjoys comedy, playing drums, watching horror films, fine cigars, and Absinthe.
DealNews may be compensated by companies mentioned in this article. Unless marked as a "Sponsored Deal," the opinions expressed here are those of the author and have not been reviewed or endorsed by the companies mentioned. Please note that, although prices sometimes fluctuate or expire unexpectedly, all products and deals mentioned in this feature were available at the lowest total price we could find at the time of publication (unless otherwise specified).
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10 comments
Amsoilrocks
Amsoil prices esp shipping out of date.

http://www.amsoil.com/offers/pc.aspx?zo=5242643
rlyoung
Thank you!
Ardbeg
Actually, I just realized I did type TOL, meaning "top of the line." Obviously that wasn't very clear if I didn't even understand it myself reading it back.
Ardbeg
rlyoung: I was just saying oil filter. Weird autocorrect mistake, sorry. What I meant by "cardboard" is that most of the cheaper oil filters (including the extremely common Fram models) have cardboard endcaps around the filter material inside and rather loose construction that may allow dirty oil to leak through (I'm sure the manufacturers would argue otherwise). The best brands (MAHLE, Purolator, others) have metal endcaps, more filter folds, etc. There are videos on Youtube where people cut them open to show construction and the differences can be striking.

HeyTiger: You'll get different opinions. Oil change shops say every there months, but that's absurd. If you're not hitting the mileage marks, I'd still say at least once a year is a good idea, perhaps more often if you live in extreme weather conditions or if you drive in very dirty air (dirt roads, smog, etc.)
HeyTiger
I have always wondered something about synthetic oil. I don't drive too often, so it takes quite a while for me to put miles on the engine. Is it important to change the oil every couple months, even if I'm not hitting the mileage?
rlyoung
Ardbeg, what's a TOL filter (I've googled)? Thanks
Ardbeg
khaino, Carfax and similar services are unreliable for maintenance records anyway (lots of stuff goes unreported, even by dealers). Keep a paper logbook in the car of maintenance (and repairs, new tires, etc.), record dates and mileage, and keep relevant receipts in the back. A smart buyer will trust that more than Carfax, anyway.

Right now, you can easily save $30+/change to change a good high-mileage/synthetic blend and a TOL filter (not the basic cardboard Fram many shops use). After an initial $80 investment (for RhinoRamps, chocks, pan, funnels, other basics like washer fluid, antifreeze, etc.) I was ahead after just 2 changes each of our cars. I was slow the first time, but now it takes me about 10 active minutes per car.

Two other advantages: (1) I'm actually using the recommended oil weights for my cars (the shops weren't) which has resulted in better gas mileage in 1 car and (2) I always have the same quality oil on hand for top-offs between changes.
mrbib
Just show them your list of oil and filter purchase receipts with mileage noted next to them.
khaino
I want to change my own oil to save money, but my concern is more of like how do I report that oil change of my own for my car history to show when people search it with the VIN?
mrbib
Yeah, I take motor oil advice from the staff writers at DealNews. For oil change intervals under 5 or 6k miles, a conventional oil is just fine. Consistency of brand is considered important by some, but all of the big brands make good oil. Havoline is a best buy much of the time as is Mobil Super.

To become a real geek, read and learn at BobIsTheOilGuy.com.
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