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Making Multi-Grade Oil

A simple standard oil, for example a pure base stock, would be a single weight, like 30 weight. This pure oil would have no detergent additives to keep the engine clean. This oil would be relatively thick and difficult to pour at room temperature, and would thin out as the motor heated up. On a very cold day, say 10° below freezing, this oil would thicken to the point where you could not start your motor, and if you did, the oil pump could not pump the oil around to protect your motor. It used to be that to start their diesel trucks in the winter, truckers would add kerosene to their oil to thin it out. Then they had to hope the kerosene would burn off before it did any real damage. Today, synthetic oils that are rated 0W-40 flow normally down to 65° below zero and remove the need for engine block heaters or adding kerosene.

An oil sold as 10w-40 is no thicker than 10 weight oil under Winter (10w) conditions, meaning below freezing. The 40 means it is no thinner than 40 weight oil at 212° Fahrenheit. So, the first number tells us the performance of the oil at or below the temperature of freezing water, and the second number tells us the performance at the temperature of boiling water. The chemicals added to the oil to accomplish this are called Viscosity Index Improvers (VIIs).

To make a 10w-40 oil, the manufacturer would start out with a 10 weight oil as the base stock. All by itself, this oil would thin out so much at normal operating temperatures that the oil film would be useless. So, they add these very special very long molecules, the VIIs. The VII molecules are as much as 1000 times as long as an oil molecule. The VII molecules curl up in a little ball at room temperature, but as the temperature gets higher they uncurl and stretch out, like a cat sleeping in the sunlight. The more stretched out the molecule is, the more it impedes the normal flow of the oil, thus raising the effective viscosity. Now, this sounds just a little too good to be true. Well, there are two catches: first, these molecules are not lubricants, so the more of them that you add the less oil you have sitting around lubricating things. Secondly, these VII molecules can be broken into pieces by various pressures and forces, like being squeezed through the transmission gears in a motorcycle or the hydraulic valves in a diesel engine. Every time a VII molecule gets broken, the oil loses some of its high temperature viscosity. Synthetic oils made from pure PAOs and/or Diesters typically have very few VIIs, so these oils are far less subject to viscosity breakdown due to shearing of the VII package. As a result, synthetics are far more stable in a motorcycle engine.

10w-30 oil increases its viscosity at high temperatures by a factor of three, which requires a significant amount of these VII molecules. 10w-40 oil increases its high temperature viscosity by a factor of four, which requires even more even longer molecules. 20w-50, which sounds a lot like 10w-40, only increases its high temperature viscosity by a factor of two and a half, so it requires fewer of these molecules than even 10w-30. 15w-40 also increases its high temperature viscosity by about two and a half, so this oil is also substantially more stable than 10w-40. Most passenger car oils today use inexpensive VII molecules that break apart relatively easily. Conversely, most diesel engine oil VIIs are chosen from more expensive chemicals that are more shear stable, since an oil change in a large diesel is expected to last for 15,000 to 150,000 miles.

One way to judge the VII content of your oil is to read the VI, the Viscosity Index, at the manufacturer's web page. The base oils all have similar VIs to start with, so generally speaking, the higher the VI in the blended oil, the more VIIs are present, and the less suitable the oil is for motorcycle usage. John Evans did just such a survey of Valvoline, Chevron, Exxon, Quaker State, Citgo, and Conoco oils. He found that the 5w-30 oils all had VI's in the range of 158-162; the 5w-20 oils had VIs of 148-154; the 10w-40 oils had VIs of 147 to 150; 10w-30 oils had VIs of 134 to 139; and 20w-50 oils had VIs of 120 to 125.

In 1994, Dr. John Woolum tested the viscosity of several 10w-40 oils in his motorcycle. He found that all of the petroleum oils had lost highly significant amounts of viscosity within 1500 miles. Only Mobil-1 held up in his test. I have personally tested Delvac-1 synthetic in my ST1300. It was 5w-40 when I put it in, and 5w-25 9,200 miles later. By 1500 miles, the petroleum oils Dr. Woolum tested were at 10w-25 equivalent. By contrast, Dr. Woolum tested a petroleum oil in his Honda Accord. After 3600 miles, the 10w-40 oil was 10w-37 equivalent. Motorcycles are indeed significantly harder on their oils than cars. Based on this result and the VI numbers above, it would seem that 10w-40, 5w-20, and 5w-30 oils cannot be safely used in motorcycles for more than 1,000 to 1,500 miles.

You might ask, if these viscosity index improvers are so expensive and fragile, why have them? Why not just run a straight 30 weight oil? If you live somewhere where the temperature never changes, like Maui, maybe that's a good idea. However, if your engine will ever see temperatures below 60 degrees or above 100 degrees, it's important to have a multi-weight oil. Multi-weight oils offer far superior protection during a cold engine start on a cold morning, and they also offer superior protection if your engine oil ever gets above about 230°. Of course, some old timers will tell you, "I always ran straight 50 weight oil. Yup. That was the stuff. All these new-fangled fancy oils, forget it, it's just marketing hype. All you need is straight 50 weight." Well, that may have been true when motorcycles were 1500ccs and made 18hp. Today, when you can casually buy an engine that makes 150hp per liter, things are just a little different.

Viscosity is not actually measured in "weights", but rather in units called "Stokes." If you're a famous scientist they name a unit after you, except for poor Albert who is considered famous enough all by himself. Stokes was a guy who worked on fluid flow. For oils, we use a hundredth of a Stoke, called a centi-Stoke, abbreviated cSt. "Weights" are a classification invented by the American Petroleum Institute (API). A different unit of viscosity, the centi-Poise, is used at very low and very high temperatures. 10 weight oil refers to oils within a range of viscosities, so two different brands of 10 weight oil might actually be quite different. 75 weight gear oil is actually about the same viscosity as 10 weight motor oil. Don't ask me why, I'm not a petroleum engineer; although some might argue that I do belong in an institution.

The "High Shear" viscosity number is the one that actually correlates with oil film thickness on your bearings at operating temperature. You can see that, based on this, really there's only three choices for oil: 20; 30 or light 40; heavy 40 to 60. The cranking viscosities above show you the temperature at which you can start your engine safely. 20w oils are only safe down to about 28°f. 5W Rotella or Mobil-1 SUV oil are safer at -13°f than 20W is at 28°f.