Interesting facts about oil

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04-22-2006, 12:30 PM

A4Driver

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Interesting facts about oil

I found these facts at a site called trustmymechanic.com and it taught me a few things I didn't know. There is a Motor Oil Bible e-book at the bottom of the page that you can download free (it is a pdf file). Here is an excerpt from it:

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 based upon the kinematic viscosity 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 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. A multi-viscosity oil also has to meet a "High Temperature/High Shear" requirement, but I'll talk about that in a minute. The 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. 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. Results from the Cold Crank Simulator (CCS) and Mini-Rotary Viscometer (MRV) tests are used to determine the 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, a 0W grade oil must have a maximum CCS centipoise (cP) value of 3250 @ -30 degrees C as well as a maximum MRV cP of 60,000 @ -40 degrees C. A 5W grade oil must have a maximum CCS cP value of 3500 @ -25 degree C and a maximum MRV cP of 60,000 @ -30 degrees C. The lower the cP value for both specifications, the better. Notice that the 0W grade oil is tested at a lower temperature on both tests AND must still have a lower CCS cP value than a 5W oil which is tested at a higher temperature. As a result, a 0w30 will allow your vehicle to start easier on a cold morning than a 5w30 will. Likewise, a 5w30 oil will pump easier in cold temperatures than a 10w30 oil will. Nevertheless, at 100 degrees C, they all fall within the same kinematic viscosity range. Therefore, they are all classified as SAE 30 weight oils at 100 degrees C. In other words, after your engine has warmed up, a 0w30 and 10w30 motor oil are basically the same thickness (within a certain SAE specified range). Of course, although this is true when the oil comes out of the bottle, we'll see in the next section that, with petroleum oils at least, the viscosity that comes out of the bottle may not necessarily be the viscosity that you find within your engine after a short period of driving.