The past couple decades have seen increases in both the quality of base oils used in passenger car motor oils and in the portions of those oils that are used. Whereas most were once made with only Group I base stock, a majority of PCMO formulas now include Group II and/or Group III. Moreover, the portions of Group II and III – and in some cases Group IV polyalphaolefin – have steadily risen, at least for the highest performing specifications.
The trend will probably continue in coming years, according to one industry consultant. H. Ernest Henderson, of K&E Petroleum Consulting LLC, told the Base Oils and Lubricants Middle East conference in May that upcoming standards will continue to require base stocks with higher viscosity index and lower volatility.
Highly saturated oils – your Group IIIs and Group III plusses – are very good for that, he said at the event, which was hosted in Manama, Bahrain, by the Conference Connection and Petrosil.
Henderson, who is based in Oklahoma City, United States, said the shift to higher grades of base oils came about because of automakers demanded improvements in several aspects of engine oil performance. Until the past couple decades, he said, the main concern for oils was that they protect engine components from wear. The easiest way to do that, he explained, was to use a thick base stock that would form a film between moving parts, thereby cushioning contacts.
A good oil was a thick oil, he said, and people didnt worry about fuel economy or low-temperature performance.
As a result, in the 1990s, most engine oils were 10W multigrades or heavier. Since then, automakers, along with regulators and consumers, have pushed for improvements along several parameters. They wanted engine oils to help improve fuel economy – both for environmental and economic reasons. Formulators accomplished this by reducing the viscosity of base stocks, and thus of finished oils. Industry introduced 5W-30 oils, then 5W-20 and has since moved to 0W multigrades. North Americas ILSAC (the International Lubricant Standardization and Accreditation Committee) is preparing to begin work on its next PCMO upgrade, GF-6, and is considering defining the lowest grade of oil yet, 0W-16.
Automakers have also been pressed to reduce emissions of air pollutants. Improved fuel economy helps with this, but they also required oil companies to reduce levels of sulfated ash, phosphorus and sulfur, collectively referred to as SAPS, so as to avoid damage to new emissions control systems. Lower volatility also reduces emissions. OEMs and consumers wanted oils that lasted longer – something achieved through greater oxidative stability.
The upshot of all these developments was that engine oils needed base stocks that were lighter, less volatile, had tighter chemical limits and greater oxidation stability. There is a basic conflict between the goals for viscosity and volatility because lighter oils tend to contain more aromatic molecules and therefore to be more volatile. Put another way, one could reduce volatility by use of heavier base stocks, but this would tend to hurt fuel economy.
Low viscosity and low volatility are now required to meet product specs, Henderson said.
The industry solved this problem by developing base stocks with higher viscosity indices. Oxidation stability comes from high levels of saturates – which also causes high VI without increasing viscosity. High VI also allows simultaneous improvements in low- and high-temperature performance.
By definition, Group I and II oils have VIs of 80 to 119, while Group IIIs are mineral oils with VI of 120 or higher. When the groups were defined in the mid-1990’s, there were almost no base stocks with VI between 110 and 120 and only a handful above that range. But afterward several refiners introduced stocks with VIs approaching 120 – these were loosely referred to as Group II+. Later a number of refiners introduced new Group IIIs – some made with hydro-cracker bottom feedstocks – with VIs in the mid to upper 120s. A few more began using waxy feeds to achieve extremely high VIs in the upper 130s or into the 140s. The last group includes gas-to-liquids base stocks and has earned the label Group III+.
Henderson said these new stocks were developed specifically for engine oil applications and have been gobbled up for them. Whereas high-performance 10W oils of the 1990s could be formulated with less than 10 percent Group III base stock, OEM specs for 5W-30s and 5W-20s required as much as 30 percent and 50 percent, respectively, he said. A typical 0W-30 may contain 100 percent Group III, while the next generation, for example a 0W-16 oil, could well need all Group III and Group IV.
Engine oils now demand the highest quality base stocks, Henderson said. Low viscosity and low volatility are now required to meet product specs. GTL base oils are ideally suited to meet this need.