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Since Ive been writing this column, I have regularly addressed the issue of fuel economy and the part that engine oil viscosity plays. Ive also raised the question of whether engine oil viscosity could drop to the point where more boundary (metal-to-metal) conditions will occur, resulting in greater engine wear and correspondingly shorter equipment life. To date, these concerns have not been born out.

The issue of lower-viscosity engine oils now has resurfaced, as the subject of an SAE Engine Oil Viscosity Classification Task Force. During Decembers ASTM Committee D-2 meeting in Anaheim, Calif., attendees heard proposals for lower High Temperature/High Shear (HTHS) viscosity limits related to expanded SAE viscosity grades. These reflect the desires of some original equipment manufacturers to use lighter viscosity oils to get maximum fuel economy.

Where is this latest effort going to lead? Will we be able to get the incremental fuel economy gains the OEMs hope for, or will we wind up with engines failing prematurely? How will it affect base oil supply? And could that make Valvolines engine warranty program the best deal ever, since it promises to cover any users engine that fails before 300,000 miles for full-synthetic oil and 150,000 miles for its standard product?

Lets take a look at what is being proposed and see where it takes us. Start with the table on page 8, which summarizes the high-temperature specifications for the current SAE engine oil viscosity classification system (J300).

The thing that stands out about the table is that the W grades currently have no HTHS viscosity limits. At present, SAE J300 uses W grades to define only low-temperature properties. The single concession to high temperature is the minimum kinematic viscosity at 100 degrees C (KV@100C).

Some automakers are asking SAE for an expanded set of high-temperature viscosity grades, based on the W grades and defined not only by KV@100C but also by HTHS viscosity limits at 150 degrees C. These grades have not been officially designated but for working purposes are being called SAE 5, 10, and 15. The proposed limits for these newcomers and where they would fit into the system are also shown.

Incidentally, there have already been a few stray written references to an SAE 10W-10 grade, which seem to presuppose that such grades will become a part of the J300 system. However, these references are from blogs and websites that are not official SAE sites.

Ford Motors representatives have indicated that they are comfortable with current SAE 20 grades for the time being. They believe that the kinematic viscosity limits at 100C for the proposed new grades do need to be modified so as to prevent overlap in grade designations. One example: Imagine an oil with a 7.7 KV@100C and an HTHS viscosity of 2.3. Using the current proposal, that oil could be classified as either a 10 or 15, or even a 5. In addition, variations in kinematic viscosity will have effects on some engine systems, even though the viscosity grade is the same.

One might wonder where the HTHS viscosity limits came from. Its a good question. The limits were established based on an extrapolation using oil with viscosity index of 100 and the minimum KV@100C for each grade. Thats not too bad a method since it allows for some correlation between a viscosity grades kinematic and HTHS viscosities. The method would seem to break down however at very low kinematic viscosity values, since the V.I. calculations get pretty sloppy when viscosities drop too low.

Interestingly, the limits for KV@100C are based on conversion of Saybolt Universal Seconds viscosity limits, which first appeared about 1950 and which were established arbitrarily. If the same cor-relations are run using higher-V.I. oils, the HTHS viscosity limits will shift somewhat to higher values relative to KV@100C.

During the ASTM meeting, Chris May of Imperial Oil, who chairs the SAE task force, pointed out that the kinematic viscosities for the proposed SAE 5 and 10 grades are relatively low in comparison to an extrapolation of the rest of the current grades, while SAE 20W seems to fit. One concern is that since the viscosity standards are based on an arbitrary selection of values, it could mean that SAE XW-20 products might be on the edge protection-wise. However, since there are no reported problems with these grades to date, it would appear that current SAE XW-20 multigrades appear to offer adequate protection.

There is similar uncertainty regarding HTHS viscosity. Although the SAE 20 HTHS viscosity limit aligns well with the other grades, the proposed 5 and 10 grades fall below the line. The OEMs want lower HTHS viscosity because of its strong positive influence on viscosity related fuel economy. However, there is a point at which boundary lubrication becomes a significant part of the equation.

There have been a number of studies done over the last decades on the relation between viscosity and wear. One SAE paper from 1990 (SAE 902064, Cryvoff, Spearot and Bates) was based on an ASTM task force report. Its authors showed that as HTHS viscosity went down, so did the minimum oil film thickness. From the current SAE 20 level to the proposed 5 level, minimum oil film thickness dropped in half. Not a good start!

Bearing weight loss has also been evaluated. Two years later, another study (SAE 922342, Demmin, Girshick and Schilowitz) showed up to 500 percent more bearing weight loss from both main and connecting rod bearings when HTHS viscosity went from 2.6 cPs to 2.2 cPs. The current SAE 20 HTHS viscosity minimum value is 2.6 cPs, while 2.2 cPs falls somewhere between the proposed 15 and 10 grades. A few years before that, Bates and Toft (SAE 892154) also showed a strong correlation between reduced HTHS viscosity and increased bearing wear. This is certainly not an outcome a concerned vehicle owner wants to see in his car.

Finally, in 1993 researchers from Japan showed that reductions in HTHS viscosity result in increased piston ring and cam face wear (SAE 932782, Ohmori, Tohyama, Yamamoto, Akiyama, Tasaka and Yoshihara). Depending on engine RPM, the increases in wear could be as large as 300 percent for ring faces and 50 percent for cam faces. For the average car owner, this is yet another yellow flag warning of potential problems.

To be fair, many of these studies were done before some of the latest engine designs were introduced. New engine designs can make a difference. Lowered ring tension and roller followers on the cam do decrease wear significantly while improving fuel economy. Bearing wear, while negatively influenced by oil viscosity, may be OK.

Closer to our own day, joint research in 2005 by Ford, Infineum and Petro-Canada explored durability issues in taxicabs operating in Las Vegas – known for blistering temperatures – using both 5W-20 and 0W-20 oils. Both oils were ILSAC GF-4 types blended with API Group II and II+ base stocks, and with the same low-phosphorus (0.05 percent) additive package. Over the 100,000 miles that each cab was driven, the team found the SAE 0W-20 was equivalent to 5W-20 in durability … and presents no harm data in wear performance.

Its also worth noting that Bob Olree, retired now from General Motors, has pointed out to LubesnGreases readers that most of the U.S. market is using 5W-XX multi-grades, and that these low-viscosity oils can ease the effects of wear during start-up. He credited the move from 10W-30 multi-grades to 5W-30 as a key factor in increasing engine life, due to improved low-temperature starting and pumping. Most ring and liner wear occurs at low temperature, he commented, and lower viscosity helps reduce wear at low temperatures.

So the idea for these new, lower SAE grades has been stewing for awhile. The concept was first discussed at an SAE Open Forum in 2005. Among the issues cited for wanting such a change were a concern about consumer resistance to engine oils named SAE 0W-20 and SAE 0W-30; the fact that fuel economy gains could be made if HTHS limits were lowered; and a concern that there was inadequate coverage for W grades related to HTHS and kinematic viscosity.

With fuel economy being such a driver for change in engine design and engine oils, its no wonder that every effort to capture even marginal improvement is being made. However, as I have pointed out before, most of us wont recognize any significant fuel economy improvement for our own vehicle from the engine oils contribution. It is only when the sum of all the incremental changes is added up that these gains will be realized. Of course, the OEMs need them to help achieve their government-mandated Corporate Average Fuel Economy tar-gets, which must ramp up more than 30 percent, to 36 mpg by 2016.

A final issue to consider, one that is near and dear to my heart, is the impact of the continuing drop in engine oil viscosity on the pool viscosity of base stocks in the marketplace. To refresh everyones memory, the more multi-viscosity engine oils and the lower the grades needed, the lighter the base oil viscosity that will be required to make the engine oils. If we are still selling finished engine oils at a rate of 1 billion gallons per year, then base stock viscosity in general will need to go down in order to accommodate these lower viscosity engine oils.

My best guess is that pool viscosity for engine oils possibly will drop another 0.5 cSt, slipping from an estimated 5.9 cSt in 2005 to around 5.4 cSt as soon as this year. The introduction of even lower-vis engine oils would undoubtedly continue this shift and perhaps make it even larger.

Whether or not these changes to SAE J300 will occur as proposed is not known at this time. One thing I do know is that the old adage about viscosity being the single most important property of an oil still holds true. It has just become a lot more complicated than a single viscosity test.

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