Otherwise, he said during a Nov. 9 presentation here at the National Petrochemical and Refiners Associations International Lubricants and Waxes Meeting, people may be surprised to find that lubricants with different base oils have significantly different performance.

Its not to say we cant get to broader interchange rules, but we need to bring the data, said X B Cox, senior planning advisor for ExxonMobil Lubricants and Specialties Co. As engines get more severe, and test criteria more rigorous, variations in base stock performance may become more pronounced.

The American Petroleum Institute is responsible in North America for licensing motor oils that meet industry specifications for passenger cars and heavy-duty diesel engines. The organization also adopts base oil interchange and read-across rules that determine the extent to which substitutions can be made for base stocks in approved formulation.

In some cases, a formulator may be able to switch completely to a different base oil without having to repeat most of the expensive engine tests needed to gain certification for the finished lubricant. In others, the rules may allow substitution of only a portion of the original base stock and still require a number of tests to be repeated.

Interchange rules for Group I and Group II oils are more extensive – and therefore allow more flexibility – than those for Group III oils, primarily because Group I and II were in broader use, and had more test data available when the interchange rules were written. Group I use is largely precluded, though, by the latest specification upgrades for both passenger car and heavy-duty engine oils. At the same time, use of Group III – like Group II – is on the rise. Some suppliers and additive companies support more lenient interchange rules because it would reduce testing costs of using their products.

Because they are made by processes designed to eliminate contaminants and to produce more uniform molecules, Group IIIs are generally viewed as more consistent and predictable than lower grades. Cox pointed out, however, that Group IIIs are made from a variety of processes and using a number of different feed stocks and as such can have a broad range of molecular composition.

To illustrate the point, he cited experiments by ExxonMobil to separate fractions of several Group III stocks by viscosity index (VI). One sample had an overall VI above 140 and its fractions ranged from 110 to 160 VI. Only ten percent of the sample had molecules whose VI was below the API Group III definition of 120. On the other hand, another Group III with an overall VI of 125, had fractions ranging from 0 to 160, and forty percent of the molecules had a VI of less than 120.

This and other differences in composition can affect performance in important ways, Cox said. ExxonMobil compared the performance of Group III oils in two different tests designed to determine motor oil tendency to form deposits on pistons and cylinders. One “high-performance” 5W-40 engine oil was blended using seven different Group III base oils.

Not only did the oils show significantly different performance within a test – the worst oil had almost twice the deposits of the best oil – but the order of performance of the Group III oils was significantly different in the two tests. For example, the oil with the worst performance in one test was among the best performers in the second test. Cox added that some of the scores would not have met requirements for certification.

ExxonMobil also conducted experiments to gauge base stock sensitivity to different additive packages. Researchers ran deposit tests on two Group III stocks formulated with each of three additive packages from different suppliers. Again results varied widely. More surprising, Cox said, was the fact that one base stock performed significantly better with one additive package, but worse with the second, and about equivalent with the third.

Cox said ExxonMobils experiments show that Group III oils are not as interchangeable or predictable as many may have assumed, at least with the current additive packages.

It says that basically not all base stocks are created equally and that additive packages have different responses to different Group III base stocks, he said. He urged the industry to move cautiously with Group III interchange rules to avoid guidelines that allow licensing of substandard engine oils.

A lot more work is required. We are just beginning to scratch the surface.