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Since 1906, automotive engine oils have evolved from straight base oil to todays fully formulated fluids, with detergents, dispersants, anti-wear, antifoam, antioxidants and friction modifiers. Last months column reviewed the history of the SAE Viscosity Classification System (now known as SAE J300), and how these products took shape over the past century. Within that story is the change from monograde engine oils to multigrades – and the relentless march to lower and lower viscosity engine oils.

This months column will explore the impact of lowered finished oil viscosity on base oil production and where its leading us. Ill look at pool viscosity (the average base oil viscosity for all engine oils) and how it has changed over the last 25 years. Ill also try to project where this is leading and what it means.

Using data going back to 1980, published by the National Petrochemical and Refiners Association, I was able to gauge the change in base oil viscosity from then to now. Prior to 1980, the impact of base oil viscosity was not as significant. There were a few very low viscosity base oils in the market, but the requirements for engine oils were such that these oils werent used very much.

The table above is the same one presented last month – Best-selling SAE Grades, 1980 to 2005 – with a key addition. Shown here are the main base oil viscosities needed to make each grade. These values are the result of older formulation data but they are fairly representative of the actual base oil viscosities currently being used to formulate engine oils.

Using the viscosities and volumes of each grade, I calculated an average pool viscosity for each year shown. Those numbers then could be graphed to demonstrate what changes occurred over time (below). The graph shows both the actual viscosity data (blue line) and a trend line for pool viscosity (black) based on that data. Looking at base oil pool viscosity from 1980 to present, the change is dramatic. Average base oil viscosity has dropped by more than 17 percent.

Shifting Needs

Since automotive engine oils represent about half of the lubricant products sold in North America annually, this shift in engine oils base oil viscosity requirements has affected all product lines. As pool viscosity dropped, the demand for lighter-viscosity base oils increased. Refiners responded by shifting the cut points in their plants to take a greater portion of the middle and heavier viscosity cuts and move them into the lighter viscosities. That resulted in less heavy-neutral and bright stock being made. Heavy-neutral is a high value material for industrial engine oils, and bright stock is widely used in industrial and gear applications calling for high viscosity, thermally stable base stocks.

New refining techniques have been introduced over the last 25 years, as well. In North America in the early 1980s, hydrocracking and hydrotreating were only used by Sun Oils lube plant in Puerto Rico andGulf Canada (later part of Petro-Canada). Today, the Sun plant is gone – but Chevron, Motiva, Calumet, Petro-Canada and ConocoPhillips (which shares its Excel Paralubes refinery output with Flint Hills Resources) have major plants on the continent producing high-quality base oils by these hydrogen processes. ExxonMobil, Imperial Oil and Valero are also producing hydrogen-processed base stocks in addition to more traditional solvent-refined base stocks.

The United States is also importing base stocks from Asia, mostly from Korea and Singapore, which are another step up on the processing ladder. These API Group III base stocks are so refined that they are virtually indistinguishable from some synthetic hydrocarbons. Of course, actual synthetic base stocks (most notably polyalphaolefin) are also being used in some of the lowest viscosity grades.

Volatile Issues

Viscosity is not the only driver for these changes. Volatility – engine oil loss due to evaporation – has become an important part of the puzzle. In 1980 there were no real volatility limits on engine oils. The only checkpoint was oil consumption in ASTMs Sequence IIIC engine test. The engine ran hot (with an oil temperature of 150 degrees C) and occasionally a test consumed too much oil due to oil volatility. In those cases, it did not constitute a test oil failure as such, but instead was deemed to be a mechanical failure – attributable to leaking seals, for example. In a few cases, the engine oil was blended with very light neutral (4 cSt @ 100 C or lower) and these oils tended to volatilize during the test run. In fact, the engine test cell sometimes had a blue haze in it due to the oil vapors being released by the engine!

By narrowing the base oil cuts, the engine oil volatility can be improved. However, the improvement comes at a price: reduced yield. The trick is to cut just enough off the front end of the base oil cut to meet volatility limits, without raising viscosity to the point where the viscosity grade cannot be made in reasonable volumes. ExxonMobil, Calumet, Imperial Oil and Motiva are examples of refiners who are producing so-called Group II+ base stocks that balance these volatility and viscosity requirements without seeming to suffer from intolerable yield losses. This is also where true synthetics are finding a home.

The volume of base oils produced in North America is adequate for current engine oil demand but it is tight. The demand for base oils for other applications (automatic transmission fluid, tractor and hydraulic fluids, gear oils, industrial lubes, greases, etc.) places a lot of pressure on refiners to produce the cuts needed for engine oils and still provide appropriate base oils for other uses.

Remember, too, that several companies are producing fully synthetic engine oils utilizing PAO and synthetic esters. Some of these products have been in the marketplace for a number of years and have an established following. They offer low volatility and low viscosity for the latest engine needs as well as meeting current engine oil category requirements. However, they are more costly than mineral oil based products and have somewhat limited market appeal.

Shear Madness

So the question remains: How low can we go?

Not so long ago, in the late 80s to early 90s, engine design was still being done using an engine oil viscosity of more or less 6 cPs at operating temperature. That would correspond to an SAE 30. However, the press for better fuel economy was driving oils down to the 3 cPs region. The industry struggled with high-temperature/high-shear viscosity limits in SAE J300 and finally settled in 1993 on a series of minimum values for non-winter grades that all felt should provide adequate protection.

Industry still requires a shear-stability test to be run and specifies that engine oils stay within the viscometric limits of their designated grade. That sets a lower limit for HTHS viscosity and regulates the amount of viscometric advantage any grade can offer. Hence, the reason for the auto industrys push to lower viscosity grade engine oils. Remember that multi-viscosity engine oils under high shear rate conditions tend to revert to base oil viscosity due to temporary viscosity loss due to shear.

If nothing intervenes to change this pattern, the ramifications of this scenario are pretty apparent. Refiners will continue to increase their production of lower-viscosity, reduced volatility base stocks until there is nothing left except 3 cSt to 5 cSt neutral. Most of the middle and high-viscosity cuts will be gone, or severely limited in volume, and will result in additional pressure on specialty products such as natural gas engine oils and certain industrial products. Synthetics, including Group III, Group IV (PAO) and Group V (others), will become more prevalent in the marketplace. The cost of engine oil will go up rather drastically. Oil drain intervals will be stretched in order to conserve and with that will come even more heavily formulated engine oils. The spiral will get tighter and tighter until someone cries, Enough, already!

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