When speaking of the properties of a motor oil, viscosity is of tremendous importance. Unfortunately, it also an area that is very confusing. The viscosity of an oil refers to its relative resistance to flow at differing temperatures. As has been previously mentioned, an oil must not only be "thin" enough to flow well at low temperatures, it must also remain "thick" enough to maintain adequate protection at high temperatures.

Ideally, an oil will maintain a consistent viscosity over a wide temperature range. The viscosity index of an oil is a measure of its tendency to change viscosity with temperature changes. The higher the viscosity index (VI) the more consistent an oil's viscosity is with temperature changes.

Monograde Oils

Monograde oils such as 30 weight oils are designed for consistent temperature applications. For instance, you will find that most older lawn tractors and mowers call for a straight 30 weight oil (SAE 30). This is because it is assumed that these will be operating mainly in warm temperature summer months.

So, if you take a look at their viscosity index, you'll notice that most monograde oils have a low VI number. This implies that as you cool the oil it will thicken quite a bit. However, this is ok, because the oil is designed to only be used under warm conditions. Cold temperature thickening will not be an issue.

According to SAE J300 standards, to be classified as a certain SAE viscosity, an oil is heated to 100 degrees C (212 degrees F). It's kinematic viscosity at this temperature is measured. If it falls within a certain range it is classified as a particular viscosity. For instance, an SAE 30 oil must have a kinematic viscosity at 100 degrees C of between 9.3 and 12.5 cSt (centistokes).

Multi-Viscosity (Multi-Grade) Oils

Multi-viscosity (multi-grade) oils such as 0w30, 5w30, 10w40 and so on are oils, which are designed for applications where temperature changes may be significant. For instance, multi-viscosity oils might be used in northern US climates where temperatures can be -20 degrees F in the winter and +95 degrees F in the summer.

However, that does not mean they cannot be used for applications where the temperature remains more consistent. The fact is, monograde oils are becoming much less common as multi-viscosity oils are being substituted in applications which traditionally called for a monograde oil.

Nevertheless, monograde oils are still used in many super high performance racing applications, construction equipment that is used in only summer months and industrial engines that are kept indoors at a constant temperature all year round.

What Do the Numbers Mean?

Most people believe that a 5w30 oil is good for cold weather use because it is a "5 weight" oil in cold temperatures and a "30 weight" oil at high temperatures. On the surface this might seem to make a certain amount of sense. Naturally, a "5 weight" oil would flow better than a "30 weight" oil. This would make it ideal for cold temperature operation.

Nevertheless, this is a profound misunderstanding of what the labeling means. The two numbers really have little to do with each other. The final number is based upon the kinematic viscosity of the oil at 100 degrees C, as we discussed for monograde oils.

So, if a multi-grade oil, when heated to 100 degrees C, falls within a certain kinematic viscosity range it is classified as a certain SAE grade (the last number - like the "30" in 5w30). In other words, the kinematic viscosity of a 5w30 multi-viscosity oil falls within the same range at 100 degrees C as a monograde SAE 30 weight oil does.

5w is NOT 5 "weight"

In contrast to a monograde oil, a multi-viscosity oil also has to meet a "High Temperature/High Shear" requirement, but I'll talk about that in a minute. Let's talk about the "w" number for a moment. This first number (the "5" in 5w30) is only a relative number which basically indicates how easily it will allow an engine to "turn over" at low temperatures. It is NOT a viscosity reference. In other words, a 10w30 is NOT a 10 weight oil in cold temperatures and a 30 weight oil in warm temperatures.

In fact, since SAE viscosity classifications only apply to an oil at 100 degrees C, it doesn't even make sense to label it as a certain SAE viscosity at any temperature other than 100 degrees C.

Besides, if you thought about it for a second, it wouldn't make sense for a 10w30 oil to be a 10 weight oil in the cold and a 30 weight oil in warm temperatures. What liquid do you know of that gets "thicker" as its temperature increases or "thinner" as the temperature decreases?

I would venture to say you probably can't come up with one. This holds true for motor oil as well. If a 10w30 was a 30 weight oil at 100 degrees C and a 10 weight oil at cold temperatures, that would mean it "thinned out" as the temperature dropped. That just doesn't make any sense considering what we know about liquids. It just doesn't happen like that.

The fact is that a 5w30 motor oil is thicker in cold temperatures than in warm temperatures. In fact, you could easily demonstrate this for yourself. Have you ever tried to pour oil out of the bottle in the winter - even a winter rated multi-viscosity oil? It pours more slowly, doesn't it? That's because the cold temperature "thickens" the oil.

However, a 5w30 motor oil will be thinner than a 10w30 motor oil when subjected to the same low temperature conditions - because the "W" number is lower. This is an indication of better cold weather performance. In other words, a 5w30 flows better in cold weather than a 10w30 motor oil will. Think of the "W" as a "winter" classification instead of a "weight" classification.

An Interesting Video

Although not, technically, "scientific", this video and some others produced by the same Youtube user show some pretty distinct differences in cold temp flow between 0w and 5w motor oils. In addition, it also shows a clear difference between a synthetic 5w and petroleum 5w oil. Of course, we don't know that all variables have been properly accounted for, and there are a limited number of brands included in this "test", so it's not exactly conclusive, but it is interesting to be able to visually see the differences. Take a look:


Notice how, if you pause the video and move it slow motion through the initial stages, the two 0w multi-viscosity oils clearly beat the 5w oils in terms of flow. However, what I find even more interesting is how little the difference is between the two 0w synthetic oils and the 5w synthetic oil. If the video is watched at full speed, the difference is almost imperceptible.

On the other hand, the difference between the synthetic 5w30 oil and the petroleum 5w30 oils is extreme. At the same time, the synthetic 5w40 oil performs no better than the petroleum 5w30 oils "tested".

Tough to Draw any Solid Conclusions

The thing is, it's tough to make any generalized conclusions from this. One reason is that two 5w30 weight oils (or ANY two oils of the SAME SAE viscosity) may actually be considerably different in terms of their actual KINEMATIC viscosity. For instance, of the 174 oils listed in the 5w30 viscosity category in our comparison charts, the kinematic viscosity at 100 degrees C ranges from 9.3 cSt to 12.3 cSt, which is a pretty wide margin.

To put that into perspective there were 16 oils which were equal to or over 12.0 cSt, and a kinematic viscosity of 12.5 cSt would move those oils into the 40 weight category. Likewise, there were 15 oils that were equal to or under 10.0 cSt and a kinematic viscosity of 9.2 cSt would move those oils into the 20 weight category.

Thus, it's not as if all of these multi-viscosity oils have the exact same viscosity characteristics, even though all 174 of them are classified as SAE 5w30 oils. In truth, those near 10.0 cSt or below are closing in on "20 weight" classification, while those over 12 cST are nearly "40 weight" oils, meaning there is close to a TWO VISCOSITY GRADE jump from those at the low end to those at the high end.

What I'm trying to say here is that, although videos like the one above are interesting to see, and may at least give you SOME idea of how specific oils compare to other specific oils that were actually "tested" for cold temp properties, don't make any over-generalizations from these "tests" there are two many variables not accounted for that could potentially shed a completely different light on the "results".

The information in the section below also provides additional reasons why the oils in the above "test" had such varying degrees of cold weather performance, even though many of them had the same 5w winter viscosity classification.

Classifying the "W" Rating

Results from the Cold Crank Simulator (CCS) and Mini-Rotary Viscometer (MRV) tests are used to determine a multi-viscosity oil's "W" grade. The better the engine "startability" of the oil at low temperature, the lower the W classification. Each W grade must meet certain "startability" requirements at a specified temperature.

For instance, ...

Motor oil viscosity is an extremely involved topic that confuses most auto owners AND mechanics, and what you've read so far on this page only scratches the surface (even though it's a really good start).

If you've made it this far you obviously have a significant interest in learning all you can about motor oil, which is a good thing. Most auto owners make a lot of easily avoidable mistakes when choosing the right motor oil & filters for their vehicles. But, you're clearly smarter than "most auto owners". You understand the value of knowing all you can about motor oil and filtration or you wouldn't have read this article all the way to the end.

Being educated about motor oil & filtration is the single most important thing you can do to make sure your vehicles last as long as possible, saving you the maximum amount of money. That's because motor oil is the lifeblood of your engine, so it pays to know what motor oil and filter will be best for YOUR vehicle.

And knowing HOW to make that decision for each vehicle in your "fleet" is FAR more valuable than asking your "Uncle Joe" what he thinks you should be using. Chances are he actually doesn't know. Just because he knows alot about engines does NOT mean he knows a thing about motor oil - even if he THINKS he does.

By tomorrow you could know more about motor oil, viscosity and filtration than about 99% of the people on the planet (including your "Uncle Joe" and all your buddies). And, knowing this information could easily save you TENS of THOUSANDS of dollars in automotive repair, maintenance and replacement costs over your lifetime.